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

[Nikita Popov via llvm-commits]

nikic wrote:

Please address https://github.com/llvm/llvm-project/pull/115566#discussion_r1847990774 before attempting another reland.

https://github.com/llvm/llvm-project/pull/117044