[LSR] Rewrite IV match for term-fold using existing utilities

Main benefit here is making the logic easier to follow, slightly more efficient, and more in line with LFTR.  This is not NFC.  There are three semantic changes here.

First, we drop handling for constants on the LHS of the comparison.  These are non-canonical, and we're very late in the optimization pipeline here, so there's no point in supporting this.  I removed a test which covered this case.

Second, we don't need the almost dead IV to be an addrec.  We just need SCEV to be able to compute a trip count for it.

Third, we require a simple IV for the almost dead IV.  In theory, this removes cases we could have previously handled, but given a) zero testing and b) multiple known correctness issues, I'm adopting an attidute of narrowing this down to something which works correctly, and *then* expanding.