[Atomics][LoopIdiom] Recognize unordered atomic memcpy

Summary:
Expanding the loop idiom test for memcpy to also recognize unordered atomic memcpy.
The only difference for recognizing
an unordered atomic memcpy and instead of a normal memcpy is
that the loads and/or stores involved are unordered atomic operations.
Background:  http://lists.llvm.org/pipermail/llvm-dev/2017-May/112779.html

Patch by Daniel Neilson!

Reviewers: reames, anna, skatkov

Reviewed By: reames

Subscribers: llvm-commits, mzolotukhin

Differential Revision: https://reviews.llvm.org/D33243

llvm-svn: 304310