[MemorySSA] When applying updates, clean unnecessary Phis.

Summary: After applying a set of insert updates, there may be trivial Phis left over. Clean them up.

Reviewers: george.burgess.iv

Subscribers: jlebar, Prazek, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63033

llvm-svn: 363094