[X86] Make the post machine scheduler macrofusion-aware.

Summary:
Given that X86 does not use this currently, this is an NFC. I'll
experiment with enabling and will report numbers.

Reviewers: andreadb, lebedev.ri

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60185

llvm-svn: 357568