[analyzer] Refactor FindLastStoreBRVisitor to not find the store ahead of time.

As Anna pointed out to me offline, it's a little silly to walk backwards through
the graph to find the store site when BugReporter will do the exact same walk
as part of path diagnostic generation.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@162719 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed