[RewriteStatepointsForGC] Workaround a lack of determinism in visit order

The visit order being used in the base pointer inference algorithm is currently non-deterministic.  When working on http://reviews.llvm.org/D12583, I discovered that we were relying on a peephole optimization to get deterministic ordering in one of the test cases.  

This change is intented to let me test and land http://reviews.llvm.org/D12583.  The current code will not be long lived.  I'm starting to investigate a rewrite of the algorithm which will combine the post-process step into the initial algorithm and make the visit order determistic.  Before doing that, I wanted to make sure the existing code was complete and the test were stable.  Hopefully, patches should be up for review for the new algorithm this week or early next.

llvm-svn: 246801