[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 `premerge-monolithic-linux` running on `premerge-linux-1` while building `llvm` at step 7 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/153/builds/15035

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

```
Step 7 (test-build-unified-tree-check-all) failure: test (failure)
...
-- Configuring done
-- Generating done
-- Build files have been written to: /build/buildbot/premerge-monolithic-linux/build/runtimes/runtimes-bins
8.107 [363/58/987] cd /build/buildbot/premerge-monolithic-linux/llvm-project/clang/bindings/python && /etc/cmake/bin/cmake -E env CLANG_NO_DEFAULT_CONFIG=1 CLANG_LIBRARY_PATH=/build/buildbot/premerge-monolithic-linux/build/lib /usr/bin/python3.10 -m unittest discover
......................................................................................................................................................
----------------------------------------------------------------------
Ran 150 tests in 2.484s

OK
18.036 [41/58/1309] 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 /usr/bin/ccache /usr/bin/clang++ -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/build/buildbot/premerge-monolithic-linux/build/unittests/MIR -I/build/buildbot/premerge-monolithic-linux/llvm-project/llvm/unittests/MIR -I/build/buildbot/premerge-monolithic-linux/build/include -I/build/buildbot/premerge-monolithic-linux/llvm-project/llvm/include -I/build/buildbot/premerge-monolithic-linux/llvm-project/third-party/unittest/googletest/include -I/build/buildbot/premerge-monolithic-linux/llvm-project/third-party/unittest/googlemock/include -gmlt -fPIC -fno-semantic-interposition -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 -ffunction-sections -fdata-sections -O3 -DNDEBUG  -Wno-variadic-macros -Wno-gnu-zero-variadic-macro-arguments -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 /build/buildbot/premerge-monolithic-linux/llvm-project/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp
/build/buildbot/premerge-monolithic-linux/llvm-project/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:35:17: error: unknown type name 'LLVMTargetMachine'; did you mean 'TargetMachine'?
std::unique_ptr<LLVMTargetMachine>
                ^~~~~~~~~~~~~~~~~
                TargetMachine
/build/buildbot/premerge-monolithic-linux/llvm-project/llvm/include/llvm/Target/TargetMachine.h:77:7: note: 'TargetMachine' declared here
class TargetMachine {
      ^
/build/buildbot/premerge-monolithic-linux/llvm-project/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:42:26: error: unknown type name 'LLVMTargetMachine'; did you mean 'TargetMachine'?
  return std::unique_ptr<LLVMTargetMachine>(
                         ^~~~~~~~~~~~~~~~~
                         TargetMachine
/build/buildbot/premerge-monolithic-linux/llvm-project/llvm/include/llvm/Target/TargetMachine.h:77:7: note: 'TargetMachine' declared here
class TargetMachine {
      ^
/build/buildbot/premerge-monolithic-linux/llvm-project/llvm/unittests/MIR/DroppedVariableStatsMIRTest.cpp:43:19: error: unknown type name 'LLVMTargetMachine'; did you mean 'TargetMachine'?
      static_cast<LLVMTargetMachine *>(T->createTargetMachine(
                  ^~~~~~~~~~~~~~~~~
                  TargetMachine
/build/buildbot/premerge-monolithic-linux/llvm-project/llvm/include/llvm/Target/TargetMachine.h:77:7: note: 'TargetMachine' declared here
class TargetMachine {
      ^
3 errors generated.
30.963 [41/2/1365] Building CXX object unittests/tools/llvm-exegesis/CMakeFiles/LLVMExegesisTests.dir/X86/SnippetGeneratorTest.cpp.o
ninja: build stopped: subcommand failed.

```

</details>

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