[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350558 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed