[X86MacroFusion] Handle branch fusion (AMD CPUs).

Summary:
This adds a BranchFusion feature to replace the usage of the MacroFusion
for AMD CPUs.

See D59688 for context.

Reviewers: andreadb, lebedev.ri

Subscribers: hiraditya, jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59872

llvm-svn: 357171