Recommit rL305677: [CodeGen] Add generic MacroFusion pass

Use llvm::make_unique to avoid ambiguity with MSVC.

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.

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

llvm-svn: 305690