[MemorySSA] Extend removeMemoryAccess API to optimize MemoryPhis.
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
Subscribers: sanjoy, jlebar, llvm-commits, Prazek
Differential Revision: https://reviews.llvm.org/D57199