[CodeGen] Add generic MacroFusion pass.

Summary:
This patch adds a generic MacroFusion pass, that is used on X86 and
AArch64, which both define target-specific shouldScheduleAdjacent
functions. This generic pass should make it easier for other targets to
implement macro fusion and I intend to add macro fusion for ARM shortly.

Reviewers: craig.topper, evandro, t.p.northover, atrick, MatzeB

Reviewed By: MatzeB

Subscribers: atrick, aemerson, mgorny, javed.absar, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D34144

llvm-svn: 305677