[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 `openmp-offload-sles-build-only` running on `rocm-worker-hw-04-sles` while building `llvm` at step 8 "Add check check-llvm".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/140/builds/11176

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

```
Step 8 (Add check check-llvm) failure: test (failure)
...
/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/include/llvm/ADT/StableHashing.h:63:15: warning: unused variable ‘S2’ [-Wunused-variable]
   auto [P2, S2] = P1.rsplit(".__uniq.");
               ^
[649/681] Linking CXX executable unittests/Transforms/Utils/UtilsTests
[650/681] Linking CXX executable unittests/Analysis/AnalysisTests
[651/681] Linking CXX executable unittests/Transforms/Scalar/ScalarTests
[652/681] Linking CXX executable unittests/tools/llvm-mca/LLVMMCATests
[653/681] Linking CXX executable unittests/Target/TargetMachineCTests
[654/681] Building CXX object unittests/IR/CMakeFiles/IRTests.dir/DroppedVariableStatsIRTest.cpp.o
[655/681] Building CXX object unittests/MIR/CMakeFiles/MIRTests.dir/DroppedVariableStatsMIRTest.cpp.o
FAILED: unittests/MIR/CMakeFiles/MIRTests.dir/DroppedVariableStatsMIRTest.cpp.o 
ccache /usr/bin/c++ -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 -Iunittests/MIR -I/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/unittests/MIR -Iinclude -I/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/include -I/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/third-party/unittest/googletest/include -I/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/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-noexcept-type -Wdelete-non-virtual-dtor -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++1z -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/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp
/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:35:17: error: ‘LLVMTargetMachine’ was not declared in this scope
 std::unique_ptr<LLVMTargetMachine>
                 ^~~~~~~~~~~~~~~~~
/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:35:17: note: suggested alternative: ‘LLVMTargetDataRef’
 std::unique_ptr<LLVMTargetMachine>
                 ^~~~~~~~~~~~~~~~~
                 LLVMTargetDataRef
/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:35:34: error: template argument 1 is invalid
 std::unique_ptr<LLVMTargetMachine>
                                  ^
/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:35:34: error: template argument 2 is invalid
/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp: In function ‘int {anonymous}::createTargetMachine(std::__cxx11::string, llvm::StringRef, llvm::StringRef)’:
/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:40:12: error: cannot convert ‘std::nullptr_t’ to ‘int’ in return
     return nullptr;
            ^~~~~~~
/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:42:26: error: ‘LLVMTargetMachine’ was not declared in this scope
   return std::unique_ptr<LLVMTargetMachine>(
                          ^~~~~~~~~~~~~~~~~
/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:42:26: note: suggested alternative: ‘createTargetMachine’
   return std::unique_ptr<LLVMTargetMachine>(
                          ^~~~~~~~~~~~~~~~~
                          createTargetMachine
/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:42:43: error: template argument 1 is invalid
   return std::unique_ptr<LLVMTargetMachine>(
                                           ^
/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:42:43: error: template argument 2 is invalid
/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:43:19: error: ‘LLVMTargetMachine’ does not name a type; did you mean ‘createTargetMachine’?
       static_cast<LLVMTargetMachine *>(T->createTargetMachine(
                   ^~~~~~~~~~~~~~~~~
                   createTargetMachine
/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:43:37: error: expected ‘>’ before ‘*’ token
       static_cast<LLVMTargetMachine *>(T->createTargetMachine(
                                     ^
/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:43:37: error: expected ‘(’ before ‘*’ token
/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:43:38: error: expected primary-expression before ‘>’ token
       static_cast<LLVMTargetMachine *>(T->createTargetMachine(
                                      ^

```

</details>

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