[llvm] [SystemZ] Enable liveness reduction in pre-RA sched strategy. (PR #188823)

[Ulrich Weigand via llvm-commits]

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

This looks to be an incremental improvement from a performance perspective, and it does also simplify the code, so we should just go for it.  LGTM, thanks!

https://github.com/llvm/llvm-project/pull/188823