[llvm] Add a pass to collect dropped var stats for MIR (PR #120501)

[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/13404

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

```
Step 8 (Add check check-llvm) failure: test (failure)
******************** TEST 'LLVM-Unit :: MIR/./MIRTests/6/17' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.build/unittests/MIR/./MIRTests-LLVM-Unit-36209-6-17.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=17 GTEST_SHARD_INDEX=6 /home/botworker/bbot/builds/openmp-offload-sles-build/llvm.build/unittests/MIR/./MIRTests
--

Note: This is test shard 7 of 17.
[==========] Running 1 test from 1 test suite.
[----------] Global test environment set-up.
[----------] 1 test from DroppedVariableStatsMIR
[ RUN      ] DroppedVariableStatsMIR.InlinedAtChild
 #0 0x00000000019412b8 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.build/unittests/MIR/./MIRTests+0x19412b8)
 #1 0x000000000193ecfc SignalHandler(int) Signals.cpp:0:0
 #2 0x00007f0ac9041910 __restore_rt (/lib64/libpthread.so.0+0x16910)
 #3 0x000000000117b209 llvm::MachineModuleInfo::MachineModuleInfo(llvm::TargetMachine const*) (/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.build/unittests/MIR/./MIRTests+0x117b209)
 #4 0x000000000052cde5 (anonymous namespace)::DroppedVariableStatsMIR_InlinedAtChild_Test::TestBody() DroppedVariableStatsMIRTest.cpp:0:0
 #5 0x000000000195fd08 testing::Test::Run() (.part.1013) gtest-all.cc:0:0
 #6 0x000000000196b522 testing::TestInfo::Run() (/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.build/unittests/MIR/./MIRTests+0x196b522)
 #7 0x0000000001973177 testing::TestSuite::Run() (.part.1016) gtest-all.cc:0:0
 #8 0x0000000001973c1d testing::internal::UnitTestImpl::RunAllTests() (/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.build/unittests/MIR/./MIRTests+0x1973c1d)
 #9 0x00000000019740c1 testing::UnitTest::Run() (/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.build/unittests/MIR/./MIRTests+0x19740c1)
#10 0x00000000004d2a85 main (/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.build/unittests/MIR/./MIRTests+0x4d2a85)
#11 0x00007f0ac895a24d __libc_start_main (/lib64/libc.so.6+0x3524d)
#12 0x000000000052958a _start /home/abuild/rpmbuild/BUILD/glibc-2.31/csu/../sysdeps/x86_64/start.S:122:0

--
exit: -11
--
shard JSON output does not exist: /home/botworker/bbot/builds/openmp-offload-sles-build/llvm.build/unittests/MIR/./MIRTests-LLVM-Unit-36209-6-17.json
********************


```

</details>

https://github.com/llvm/llvm-project/pull/120501