[llvm] [RISCV] Override default sched policy (PR #115445)

[Philip Reames via llvm-commits]

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

LGTM - This looks very clearly like a good idea - to the point where I am mildly shocked we haven't done this before.  I think, in fact, that this probably fixes a bug I was chasing just yesterday.  :)

I am not too worried about the noted increase in spills as any perturbation to scheduling is going to cause changes in register allocation.  Looking through the test diffs, I see both improvements and regressions.  I don't see evidence of systematic regressions.

It'd be nice if we had perf data (and/or stats) from the BP3 (@lukel97 , @mikhailramalho ), but I don't consider that blocking as this is so clearly a good idea from first principles.  

Please do wait a day or so in case @topperc , @asb , @lukel97 or others have comments.

https://github.com/llvm/llvm-project/pull/115445