[MemorySSA] Refactor removing multiple trivial phis [NFC].

Summary: Create a method to clean up multiple potentially trivial phis, since we will need this often.

Reviewers: george.burgess.iv

Subscribers: jlebar, Prazek, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61471

llvm-svn: 359842