Make use of isAtLeastRelease/Acquire in the ARM/AArch64 backends

Summary:
Make use of isAtLeastRelease/Acquire in the ARM/AArch64 backends
These helper functions are introduced in D4844.
Depends D4844

Test Plan: make check-all passes

Reviewers: jfb

Subscribers: aemerson, llvm-commits, mcrosier, reames

Differential Revision: http://reviews.llvm.org/D4937