[MemorySSA] Remove optimized value when reseting optimized.

Summary:
If we don't reset the optimized value O for access A, even though A is no longer optimized to O, A will still show up in that O's users list.
This fails verification when hoisting a Def outside a loop, even though the updates are correct.
The reason is that the phi in the loop header still find as user the hoisted def, because the Def has a pointer to the Phi in its optimized operand.

Reviewers: george.burgess.iv

Subscribers: sanjoy, jlebar, Prazek, llvm-commits

Differential Revision: https://reviews.llvm.org/D56467

llvm-svn: 350783