[mir] Print/Parse both MOLoad and MOStore when they occur together.

Summary:
They're not always mutually exclusive. read-modify-write atomics are both
at the same time. One example of this is the SWP instructions on AArch64.
Another example is GlobalISel's G_ATOMICRMW_* generic instructions which
will be added in a later patch.

Reviewers: arphaman, aemerson

Reviewed By: aemerson

Subscribers: aemerson, javed.absar, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D40157

llvm-svn: 319202