[RLEV] Rewrite loop exit values for multiple exit loops w/o overall loop exit count

We already supported rewriting loop exit values for multiple exit loops, but if any of the loop exits were not computable, we gave up on all loop exit values. This patch generalizes the existing code to handle individual computable loop exits where possible.

As discussed in the review, this is a starting point for figuring out a better API.  The code is a bit ugly, but getting it in lets us test as we go.  

Differential Revision: https://reviews.llvm.org/D65544

llvm-svn: 368898