[LoopUnroll] Clean up exit folding (NFC)

This does some non-functional cleanup of exit folding during
unrolling. The two main changes are:

 * First rewrite latch->header edges, which is unrelated to exit
   folding.
 * Combine folding for latch and non-latch exits. After the
   previous change, the only difference in their logic is that
   for non-latch exits we currently only fold "known non-exit"
   cases, but not "known exit" cases.

I think this helps a lot to clarify this code and prepare it for
future changes.

Differential Revision: https://reviews.llvm.org/D103333