[Hexagon, SystemZ] Be super conservative about atomics

As requested during review of D57601, be equally conservative for atomic MMOs as for volatile MMOs in all in tree backends. At the moment, all atomic MMOs are also volatile, but I'm about to change that.

Reviewed as part of https://reviews.llvm.org/D58490, with other backends still pending review.  

llvm-svn: 354740