Exploit a zero LoopExit count to eliminate loop exits

This turned out to be surprisingly effective. I was originally doing this just for completeness sake, but it seems like there are a lot of cases where SCEV's exit count reasoning is stronger than it's isKnownPredicate reasoning.

Once this is in, I'm thinking about trying to build on the same infrastructure to eliminate provably untaken checks. There may be something generally interesting here.

Differential Revision: https://reviews.llvm.org/D63618

llvm-svn: 364135