This patch attempts to reland
https://github.com/llvm/llvm-project/pull/120780 while addressing the
issues that caused the patch to be reverted.

Namely:

1. The patch had included code from the llvm/Passes directory in the
llvm/CodeGen directory.

2. The patch increased the backend compile time by 2% due to adding a
very expensive include in MachineFunctionPass.h

The patch has been re-structured so that there is no dependency between
the llvm/Passes and llvm/CodeGen directory, by moving the base class,
`class DroppedVariableStats` to the llvm/IR directory.

The expensive include in MachineFunctionPass.h has been changed to
contain forward declarations instead of other header includes which was
pulling a ton of code into MachineFunctionPass.h and should resolve any
issues when it comes to compile time increase.

This reverts commit 3bf91ad2a9c75dd045961e45fdd830fd7b7a5455.
(llvmorg-20-init-16123-g3bf91ad2a9c7)

`llvm/CodeGen` should not depend on `llvm/Passes`.

This patch uses the DroppedVariableStats class to add dropped variable
statistics for MIR passes.

Reland 1c082c9cd12efaa67a32c5da89a328c458ed51c5

This reverts commit 223c7648468cd4f649a578d3f9cbc27a63523192.

Reverted due to vuildbot failure:

flang-aarch64-libcxx

Linking CXX shared library lib/libLLVMAnalysis.so.20.0git
FAILED: lib/libLLVMAnalysis.so.20.0git

Reland "Add a pass to collect dropped var stats for MIR" (#117044)

I am trying to reland https://github.com/llvm/llvm-project/pull/115566

I also moved the DroppedVariableStats code to the Analysis lib

This is part of a stack of patches with
https://github.com/llvm/llvm-project/pull/120502 being the first one in
the stack

This reverts commit 249755cedb17ffa707253edcef1a388f807caa35.

Broke https://lab.llvm.org/buildbot/#/builders/160/builds/9420

Note: This is test shard 99 of 154.
[==========] Running 2 tests from 2 test suites.
[----------] Global test environment set-up.
[----------] 1 test from DroppedVariableStatsMIR
[ RUN      ] DroppedVariableStatsMIR.InlinedAt

--
exit: -11

Moved the MIR Test to the unittests/CodeGen folder

This is patch is part of a stack of patches, and follows
https://github.com/llvm/llvm-project/pull/117042

I moved the MIR test to the unittests/CodeGen folder 

I am trying to reland https://github.com/llvm/llvm-project/pull/115566

This reverts commit 6e2b77d4696d4a672635c0ba1ead4824e2158a7d.

Reverting due to buildbot failure:

unittests/IR/CMakeFiles/IRTests.dir/DroppedVariableStatsIRTest.cpp.o:DroppedVariableStatsIRTest.cpp:function llvm::DroppedVariableStatsIR::runAfterPass(llvm::StringRef, llvm::Any): error: undefined reference to 'llvm::DroppedVariableStatsIR::runOnModule(llvm::Module const*, bool)'

This patch uses the DroppedVariableStats class to add dropped variable
statistics for MIR passes.

Replace it with a forward declaration instead. Analysis.h is pulled in
by all passes, but not all passes need to access the module.