[MemorySSA] Fix bugs in walker; refactor unittests a bit.

This patch fixes two somewhat related bugs in MemorySSA's caching
walker. These bugs were found because D19695 brought up the problem
that we'd have defs cached to themselves, which is incorrect.

The bugs this fixes are:

- We would sometimes skip the nearest clobber of a MemoryAccess, because
  we would query our cache for a given potential clobber before
  checking if the potential clobber is the clobber we're looking for.
  The cache entry for the potential clobber would point to the nearest
  clobber *of the potential clobber*, so if that was a cache hit, we'd
  ignore the potential clobber entirely.

- There are times (sometimes in DFS, sometimes in the getClobbering...
  functions) where we would insert cache entries that say a def
  clobbers itself.

There's a bit of common code between the fixes for the bugs, so they
aren't split out into multiple commits.

This patch also adds a few unit tests, and refactors existing tests a
bit to reduce the duplication of setup code.

llvm-svn: 268087

0 files changed, 0 insertions, 0 deletions