[llvm] Add a pass to collect dropped var stats for MIR. (PR #115566)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-x86_64-gcc-ubuntu` running on `sie-linux-worker3` while building `llvm` at step 5 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/174/builds/8656

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 5 (build-unified-tree) failure: build (failure)
...
444.035 [218/34/6704] Building CXX object unittests/DebugInfo/DWARF/CMakeFiles/DebugInfoDWARFTests.dir/DwarfGenerator.cpp.o
444.099 [217/34/6705] Building CXX object unittests/Target/LoongArch/CMakeFiles/LoongArchTests.dir/MCInstrAnalysisTest.cpp.o
444.208 [216/34/6706] Building CXX object unittests/Target/RISCV/CMakeFiles/RISCVTests.dir/MCInstrAnalysisTest.cpp.o
444.270 [215/34/6707] Linking CXX executable bin/llvm-jitlink
444.613 [214/34/6708] Building CXX object unittests/CodeGen/GlobalISel/CMakeFiles/GlobalISelTests.dir/InstructionSelectTest.cpp.o
444.739 [213/34/6709] Building CXX object unittests/IR/CMakeFiles/IRTests.dir/DroppedVariableStatsIRTest.cpp.o
444.849 [212/34/6710] Linking CXX static library lib/libclangCodeGen.a
444.943 [211/34/6711] Linking CXX static library lib/libclangFrontendTool.a
444.996 [210/34/6712] Linking CXX executable unittests/Frontend/LLVMFrontendTests
445.160 [209/34/6713] Building CXX object unittests/MIR/CMakeFiles/MIRTests.dir/DroppedVariableStatsMIRTest.cpp.o
FAILED: unittests/MIR/CMakeFiles/MIRTests.dir/DroppedVariableStatsMIRTest.cpp.o 
/opt/ccache/bin/g++ -DGTEST_HAS_RTTI=0 -DLLVM_BUILD_STATIC -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/build/unittests/MIR -I/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/unittests/MIR -I/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/build/include -I/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/include -I/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/third-party/unittest/googletest/include -I/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/third-party/unittest/googlemock/include -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -fno-lifetime-dse -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wno-missing-field-initializers -pedantic -Wno-long-long -Wimplicit-fallthrough -Wno-uninitialized -Wno-nonnull -Wno-class-memaccess -Wno-redundant-move -Wno-pessimizing-move -Wno-noexcept-type -Wdelete-non-virtual-dtor -Wsuggest-override -Wno-comment -Wno-misleading-indentation -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG  -Wno-dangling-else -Wno-variadic-macros -fno-exceptions -funwind-tables -fno-rtti -UNDEBUG -Wno-suggest-override -std=c++17 -MD -MT unittests/MIR/CMakeFiles/MIRTests.dir/DroppedVariableStatsMIRTest.cpp.o -MF unittests/MIR/CMakeFiles/MIRTests.dir/DroppedVariableStatsMIRTest.cpp.o.d -o unittests/MIR/CMakeFiles/MIRTests.dir/DroppedVariableStatsMIRTest.cpp.o -c /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp
/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:35:17: error: ‘LLVMTargetMachine’ was not declared in this scope
   35 | std::unique_ptr<LLVMTargetMachine>
      |                 ^~~~~~~~~~~~~~~~~
/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:35:34: error: template argument 1 is invalid
   35 | std::unique_ptr<LLVMTargetMachine>
      |                                  ^
/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:35:34: error: template argument 2 is invalid
/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp: In function ‘int {anonymous}::createTargetMachine(std::string, llvm::StringRef, llvm::StringRef)’:
/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:40:12: error: cannot convert ‘std::nullptr_t’ to ‘int’ in return
   40 |     return nullptr;
      |            ^~~~~~~
/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:42:26: error: ‘LLVMTargetMachine’ was not declared in this scope; did you mean ‘createTargetMachine’?
   42 |   return std::unique_ptr<LLVMTargetMachine>(
      |                          ^~~~~~~~~~~~~~~~~
      |                          createTargetMachine
/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:42:43: error: template argument 1 is invalid
   42 |   return std::unique_ptr<LLVMTargetMachine>(
      |                                           ^
/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:42:43: error: template argument 2 is invalid
/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:43:19: error: ‘LLVMTargetMachine’ does not name a type; did you mean ‘createTargetMachine’?
   43 |       static_cast<LLVMTargetMachine *>(T->createTargetMachine(
      |                   ^~~~~~~~~~~~~~~~~
      |                   createTargetMachine
/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:43:37: error: expected ‘>’ before ‘*’ token
   43 |       static_cast<LLVMTargetMachine *>(T->createTargetMachine(
      |                                     ^
/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:43:37: error: expected ‘(’ before ‘*’ token
   43 |       static_cast<LLVMTargetMachine *>(T->createTargetMachine(
      |                                     ^
      |                                     (
/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:43:38: error: expected primary-expression before ‘>’ token
   43 |       static_cast<LLVMTargetMachine *>(T->createTargetMachine(
      |                                      ^
/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp: In member function ‘virtual void {anonymous}::DroppedVariableStatsMIR_BothDeleted_Test::TestBody()’:
/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:185:28: error: request for member ‘get’ in ‘TM’, which is of non-class type ‘int’
  185 |   MachineModuleInfo MMI(TM.get());
      |                            ^~~

```

</details>

https://github.com/llvm/llvm-project/pull/115566