[indvars] Allow rotation (narrowing) of exit test when discovering trip count

This relaxes the one-use requirement on the rotation transform specifically for the case where we know we're zexting an IV of the loop.  This allows us to discover trip count information in SCEV, which seems worth a single extra loop invariant truncate.  Honestly, I'd prefer if SCEV could just compute the trip count directly (e.g. D109457), but this unblocks practical benefit.