Add MI-Sched support for x86 macro fusion.

This is an awful implementation of the target hook. But we don't have
abstractions yet for common machine ops, and I don't see any quick way
to make it table-driven.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184664 91177308-0d34-0410-b5e6-96231b3b80d8