[ARM] Always enable UseAA in the arm backend

[ARM] Always enable UseAA in the arm backend

This feature controls whether AA is used into the backend, and was
previously turned on for certain subtargets to help create less
constrained scheduling graphs. This patch turns it on for all
subtargets, so that they can all make use of the extra information to
produce better code.

Differential Revision: https://reviews.llvm.org/D69796