[X86][NFC] Add a `use-aa` feature.

Summary:
This allows enabling useaa on the command-line and will allow enabling the
feature on a per-CPU basis where benchmarking shows improvements.

This is modelled after the ARM/AArch64 target.

Reviewers: RKSimon, andreadb, craig.topper

Subscribers: javed.absar, kristof.beyls, hiraditya, ychen, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67266

llvm-svn: 371989