[llvm] [MachineScheduler] Experimental option to partially disable pre-ra scheduling. (PR #90181)

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

> I don't think this branch will actually land, although there may be some ideas worth trying. I have now another PR in progress that will replace this one. See #135076.
> 
> @michaelmaitland Maybe you would like to try the new SystemZPreRASchedStrategy and see if it works on your target as well?

CC @mshockwave 

https://github.com/llvm/llvm-project/pull/90181