MemorySSA: Add support for renaming uses in the updater.

Summary:
This lets one add aliasing stores to the updater.
(i'm next going to move the creation/etc functions to the updater)

Reviewers: george.burgess.iv

Subscribers: llvm-commits, Prazek

Differential Revision: https://reviews.llvm.org/D30154

llvm-svn: 295677