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

[via llvm-commits]

yueshe wrote:

Hello. The two new tests added (`misched-prera-loads.mir` and `misched-prera-pdiffs.mir`) were failing on our bots with `llc: Unknown command line argument '-debug-only=machine-scheduler'`
Wondering if it's possible to add `# REQUIRES: asserts` to the test files?

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