[MemorySSA] Add APIs to MemoryPhis to delete incoming blocks/values, and an...

[MemorySSA] Add APIs to MemoryPhis to delete incoming blocks/values, and an updater API to remove blocks.

Summary:
MemoryPhis now have APIs analogous to BB Phis to remove an incoming value/block.
The MemorySSAUpdater uses the above APIs when updating MemorySSA given a set of dead blocks about to be deleted.

Reviewers: george.burgess.iv

Subscribers: sanjoy, jlebar, Prazek, llvm-commits

Differential Revision: https://reviews.llvm.org/D48396

llvm-svn: 336015