Rename MachineMemOperand::getOrdering -> getSuccessOrdering.

Since this method can apply to cmpxchg operations, make sure it's clear
what value we're actually retrieving.  This will help ensure we don't
accidentally ignore the failure ordering of cmpxchg in the future.

We could potentially introduce a getOrdering() method on AtomicSDNode
that asserts the operation isn't cmpxchg, but not sure that's
worthwhile.

Differential Revision: https://reviews.llvm.org/D103338