[SystemZ]  Enable machine scheduler.

The machine scheduler (before register allocation) is enabled by default for
SystemZ.

The SelectionDAG scheduling preference now becomes source order scheduling
(was regpressure).

Review: Ulrich Weigand
https://reviews.llvm.org/D37977

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315063 91177308-0d34-0410-b5e6-96231b3b80d8
46 files changed