[AArch64] Enable post-RA MI scheduler.

Phabricator Revision: http://reviews.llvm.org/D5278
Patch by Sanjin Sijaric!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217693 91177308-0d34-0410-b5e6-96231b3b80d8