[DSE,MemorySSA] Handle atomic stores explicitly in isReadClobber.

Atomic stores are modeled as MemoryDef to model the fact that they may
not be reordered, depending on the ordering constraints.

Atomic stores that are monotonic or weaker do not limit re-ordering, so
we do not have to treat them as potential read clobbers.

Note that llvm/test/Transforms/DeadStoreElimination/MSSA/atomic.ll
already contains a set of negative test cases.

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D87386