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

[Cullen Rhodes via llvm-commits]

c-rhodes wrote:

I've been investigating a couple of scheduler issues recently and while it's possible to see what heuristics are driving it in debug output and with grep ("Pick "), I figured it would be good if we just had stats for this, perhaps others will also find this useful.

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