[MemorySSA] Add convenience APIs in updater to avoid needing MSSA.

Summary:
Ideally passes should not need to pass MSSA around and do all updates through the updater.
Add convenience APIs to help with that.

Reviewers: george.burgess.iv

Subscribers: sanjoy, jlebar, llvm-commits, Prazek

Differential Revision: https://reviews.llvm.org/D48334

llvm-svn: 335179