[llvm] Add a pass to collect dropped var stats for MIR. (PR #115566)

[Orlando Cazalet-Hyams via llvm-commits]

https://github.com/OCHyams approved this pull request.

Just reviewing the 3rd commit (other commit review comments over on #115563), LGTM (with one tiny inline nit to resolve).

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