[llvm] Add a pass to collect dropped var stats for MIR (PR #120780)

[NAKAMURA Takumi via llvm-commits]

https://github.com/chapuni commented:

You should've moved DroppedVariableStats stuff out of llvm/Passes in advance.
At a glance, they don't depend on LLVMPass any more. (#120711)

https://github.com/llvm/llvm-project/pull/120780