MemorySSA: Add support for caching clobbering access in stores

Summary:
This enables us to cache the clobbering access for stores, despite the
fact that we can't rewrite the use-def chains themselves.

Early testing shows that, after this change, for larger testcases, it will be a significant net positive (memory and time) to remove the walker caching.

Reviewers: george.burgess.iv, davide

Subscribers: Prazek, llvm-commits

Differential Revision: https://reviews.llvm.org/D31567

llvm-svn: 299322