Fix a bug in the definition of isUnordered on MachineMemOperand

Background: At the moment, we record the AtomicOrdering of an access in the MMO, but also mark any atomic access as volatile in SelectionDAG. GlobalISEL keeps the two separate, but currently doesn't know how to lower an atomic G_LOAD at all. See https://reviews.llvm.org/D57601 for context.

The definition used for unordered was only checking volatility, not atomicity. As noted above, all atomic MMOs are currently also volatile, so this is a latent bug only. Copy the definition used in IR, after auditing the two (2) uses of the function to be sure the desired semantics are the same.

Differential Revision: https://reviews.llvm.org/D57593

llvm-svn: 352898