[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-aarch64-darwin` running on `doug-worker-5` while building `llvm` at step 5 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/190/builds/9730

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

```
Step 5 (build-unified-tree) failure: build (failure)
...
282.874 [150/8/5793] Building CXX object unittests/Support/CMakeFiles/SupportTests.dir/raw_sha1_ostream_test.cpp.o
282.886 [149/8/5794] Building CXX object unittests/Support/CMakeFiles/SupportTests.dir/raw_socket_stream_test.cpp.o
282.893 [148/8/5795] Building CXX object unittests/Support/CMakeFiles/SupportTests.dir/xxhashTest.cpp.o
282.904 [147/8/5796] Building CXX object unittests/Support/DynamicLibrary/CMakeFiles/DynamicLibraryLib.dir/ExportedFuncs.cpp.o
282.916 [146/8/5797] Linking CXX static library lib/libDynamicLibraryLib.a
282.939 [145/8/5798] Building CXX object unittests/Support/DynamicLibrary/CMakeFiles/DynamicLibraryTests.dir/DynamicLibraryTest.cpp.o
282.963 [144/8/5799] Building CXX object unittests/IR/CMakeFiles/IRTests.dir/DroppedVariableStatsIRTest.cpp.o
282.975 [143/8/5800] Building CXX object unittests/Support/DynamicLibrary/CMakeFiles/PipSqueak.dir/PipSqueak.cpp.o
282.988 [142/8/5801] Building CXX object unittests/Support/DynamicLibrary/CMakeFiles/SecondLib.dir/PipSqueak.cpp.o
283.006 [141/8/5802] Building CXX object unittests/MIR/CMakeFiles/MIRTests.dir/DroppedVariableStatsMIRTest.cpp.o
FAILED: unittests/MIR/CMakeFiles/MIRTests.dir/DroppedVariableStatsMIRTest.cpp.o 
CCACHE_CPP2=yes CCACHE_HASHDIR=yes /opt/homebrew/bin/ccache /usr/bin/clang++ -DGTEST_HAS_RTTI=0 -DLLVM_BUILD_STATIC -D_DEBUG -D_GLIBCXX_ASSERTIONS -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/Users/buildbot/buildbot-root/aarch64-darwin/build/unittests/MIR -I/Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/unittests/MIR -I/Users/buildbot/buildbot-root/aarch64-darwin/build/include -I/Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/include -I/Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/third-party/unittest/googletest/include -I/Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/third-party/unittest/googlemock/include -isystem /opt/homebrew/include -fPIC -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-long -Wc++98-compat-extra-semi -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -O3 -DNDEBUG -std=c++17 -arch arm64 -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX14.4.sdk  -Wno-variadic-macros -Wno-gnu-zero-variadic-macro-arguments -fno-exceptions -funwind-tables -fno-rtti -UNDEBUG -Wno-suggest-override -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 /Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp
/Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:35:17: error: unknown type name 'LLVMTargetMachine'; did you mean 'TargetMachine'?
std::unique_ptr<LLVMTargetMachine>
                ^~~~~~~~~~~~~~~~~
                TargetMachine
/Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/include/llvm/Target/TargetMachine.h:77:7: note: 'TargetMachine' declared here
class TargetMachine {
      ^
/Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:42:26: error: unknown type name 'LLVMTargetMachine'; did you mean 'TargetMachine'?
  return std::unique_ptr<LLVMTargetMachine>(
                         ^~~~~~~~~~~~~~~~~
                         TargetMachine
/Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/include/llvm/Target/TargetMachine.h:77:7: note: 'TargetMachine' declared here
class TargetMachine {
      ^
/Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:43:19: error: unknown type name 'LLVMTargetMachine'; did you mean 'TargetMachine'?
      static_cast<LLVMTargetMachine *>(T->createTargetMachine(
                  ^~~~~~~~~~~~~~~~~
                  TargetMachine
/Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/include/llvm/Target/TargetMachine.h:77:7: note: 'TargetMachine' declared here
class TargetMachine {
      ^
3 errors generated.
283.026 [141/7/5803] Linking CXX executable unittests/Support/SupportTests
283.046 [141/6/5804] Linking CXX shared module unittests/Support/DynamicLibrary/SecondLib.dylib
283.050 [141/5/5805] Linking CXX shared module unittests/Support/DynamicLibrary/PipSqueak.dylib
284.055 [141/4/5806] Building CXX object unittests/MIR/CMakeFiles/MIRTests.dir/MachineMetadata.cpp.o
284.515 [141/3/5807] Building CXX object unittests/MI/CMakeFiles/MITests.dir/LiveIntervalTest.cpp.o
284.577 [141/2/5808] Building CXX object unittests/IR/CMakeFiles/IRTests.dir/PassManagerTest.cpp.o
284.925 [141/1/5809] Building CXX object unittests/DebugInfo/DWARF/CMakeFiles/DebugInfoDWARFTests.dir/DWARFDebugInfoTest.cpp.o
ninja: build stopped: subcommand failed.

```

</details>

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