[MachineScheduler] Order FI-based memops based on stack direction

It makes more sense to order FI-based memops in descending order when
the stack goes down. This allows offsets to stay "consecutive" and allow
easier pattern matching.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@347906 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed