[X86] Provide a separate feature bit for macro fusion support instead of basing it on the AVX flag

Summary:
Currently we determine if macro fusion is supported based on the AVX flag as a proxy for the processor being Sandy Bridge".

This is really strange as now AMD supports AVX. It also means if user explicitly disables AVX we disable macro fusion.

This patch adds an explicit macro fusion feature. I've also enabled for the generic 64-bit CPU (which doesn't have AVX)

This is probably another candidate for being in the MI layer, but for now I at least wanted to correct the overloading of the AVX feature.

Reviewers: spatel, chandlerc, RKSimon, zvi

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37280

llvm-svn: 312097