[llvm] [SystemZ] Add a SystemZ specific pre-RA scheduling strategy. (PR #135076)

[Jonas Paulsson via llvm-commits]

JonPsson1 wrote:

Some minor fixes. It is all looking quite good now to me so the next step would be to decide on using PressureDiffs (or not). 

Given that the PressureDiffs give identical results, they make sense to me, I guess. I also think that maybe SystemZPreRASchedStrategy::initializePressureSets() could be simplified, given that GCNSchedStrategy::pickNodeFromQueue() is hard-coding these sets by name, which is an indicator that these may be fairly stable.

https://github.com/llvm/llvm-project/pull/135076