MemorySSA: Remove MemorySSA walker caching.

Summary:
Remove all the caching the clobber walker does, and that the
caching walker does.  With the patch to enable storing clobbering
access results for stores, i can find no improvement with the cache
turned on (and a number of degradations, both time and memory, from
the cost of caching.  For a large program i have, we do millions of
lookups and inserts with zero hits).

I haven't tried to rename or simplify the walker otherwise yet.

(Appreciate some perf testing on this past my own testing)

Reviewers: george.burgess.iv, davide

Subscribers: Prazek, llvm-commits

Differential Revision: https://reviews.llvm.org/D31576

llvm-svn: 299578