[DSE] Teach the pass that atomic memory intrinsics are stores.

Summary:
This change teaches DSE that the atomic memory intrinsics are stores
that can be eliminated, and can allow other stores to be eliminated.
This change specifically does not teach DSE that these intrinsics
can be partially eliminated (i.e. length reduced, and dest/src changed);
that will be handled in another change.

Reviewers: mkazantsev, skatkov, apilipenko, efriedma, rsmith

Reviewed By: efriedma

Subscribers: dmgreen, llvm-commits

Differential Revision: https://reviews.llvm.org/D45535

llvm-svn: 330629