GlobalISel: represent atomic loads & stores via the MachineMemOperand.

Also make sure the AArch64 backend doesn't try to convert them into normal
loads and stores.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@294993 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed