[MemorySSA] Refactor CachingWalker.

Summary:
Refactor caching walker to make creating a walker that skips the
starting access strightforward.

Reviewers: george.burgess.iv

Subscribers: sanjoy, jlebar, Prazek, llvm-commits, jfb

Differential Revision: https://reviews.llvm.org/D55957

llvm-svn: 350558