[MemCpyOpt] Preserve MemorySSA.

This patch updates MemCpyOpt to preserve MemorySSA. It uses the
MemoryDef at the insertion point of the builder and inserts the new def
after that def.

In some cases, we just modify a memory instruction. In that case, get
the defining access, then remove the memory access and add a new one.
If the defining access is in a different block, insert a new def at the
beginning of the current block, otherwise after the defining access.

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D86651