[MemorySSA] Make Phi cleanups consistent.

Summary:
Make Phi cleanups consistent: remove self as a trivial Phi and
recurse to potentially remove other trivial phis.

Reviewers: george.burgess.iv

Subscribers: Prazek, sanjoy.google, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66454

llvm-svn: 369466