[llvm] [MISched] Add statistics for heuristics (PR #137981)

[Min-Yih Hsu via llvm-commits]

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

LGTM. I think these counters will be useful during development.
It's true that the sheer number of new statistic counters looks a little scary, but IMHO users could always opt-out so I think it's fine.

https://github.com/llvm/llvm-project/pull/137981