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

[Cullen Rhodes via llvm-commits]

c-rhodes wrote:

> On a related note, I've been thinking about improving the trace output of MachineScheduler because I alway found some messages a bit difficult to comprehend, I'm wondering what's your thoughts?

I think that's a great idea I also found it difficult, particularly when scheduling is enabled both pre and post register-allocation!

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