[PowerPC] Replace the Post RA List Scheduler with the Machine Scheduler

  We want to run the Machine Scheduler instead of the List Scheduler after RA.
  Checked with a performance run on a Power 9 machine with SPEC 2006 and while
  some benchmarks improved and others degraded the geomean was slightly improved
  with the Machine Scheduler.

  Differential Revision: https://reviews.llvm.org/D45265

llvm-svn: 336295