[unroll] Split full exact and full bound unroll costing [NFC]

This change should be NFC. It's posted for review mostly to make sure others are happy with the names I'm introducing for "exact full unroll" and "bounded full unroll". The motivation here is that our cost model for bounded unrolling is too aggressive - it gives benefits for exits we aren't going to prune - but I also just think the new version of the code is a lot easier to follow.

Differential Revision: https://reviews.llvm.org/D114453