[MemorySSA] Extend removeMemoryAccess API to optimize MemoryPhis.

Summary:
EarlyCSE needs to optimize MemoryPhis after an access is removed and has
special handling for it. This should be handled by MemorySSA instead.
The default remains that MemoryPhis are *not* optimized after an access
is removed.

Reviewers: george.burgess.iv

Subscribers: sanjoy, jlebar, llvm-commits, Prazek

Differential Revision: https://reviews.llvm.org/D57199

llvm-svn: 352787