[MemorySSA] Don't optimize incomplete phis.

Summary:
MemoryPhis cannot be optimized out until they are complete.
Resolves PR41254.

Reviewers: george.burgess.iv

Subscribers: sanjoy, jlebar, Prazek, jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59966

llvm-svn: 357315