[EarlyCSE & MSSA] Cleanup special handling for removing MemoryAccesses.

Summary: Moving special handling to MemorySSAUpdater in D57199.

Reviewers: gberry, george.burgess.iv

Subscribers: sanjoy, jlebar, Prazek, llvm-commits

Differential Revision: https://reviews.llvm.org/D57200

llvm-svn: 352794