Reapply "[MachineScheduler] Fix for PR21807: minor code difference building with/without -g."

This reapplies r224118 with a fix for test 'misched-code-difference-with-debug.ll'.
That test was failing on some buildbots because it was x86 specific but it was
missing a target triple.
Added an explicit triple to test misched-code-difference-with-debug.ll.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224126 91177308-0d34-0410-b5e6-96231b3b80d8