Revert "Temporarily enable MI-Sched on X86."

This reverts commit 98a9b72e8c56dc13a2617de84503a3d78352789c.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184823 91177308-0d34-0410-b5e6-96231b3b80d8