[MemCpyOpt] Fix MemorySSA preservation

moveUp() moves instructions, so we should move the corresponding
memory accesses as well. We should also move the store instruction
itself: Even though we'll end up removing it later, this gives us
a correct MemoryDef to replace.

The implementation is somewhat more complicated than it should be,
because we also handle the case where P does not have a memory
access due to a degnerate AA pipeline. Hopefully, the need for this
will go away in the future, when the rest of the pass is based on
MSSA.

Differential Revision: https://reviews.llvm.org/D88778