[cgp] improve robustness of uadd/usub transforms

LSR prefers to schedule iv increments just before the latch.  The recent 80511565 broadened this to moving increments in the original IR.  This pointed out a robustness problem with the CGP transform.

When we have a use of an induction increment outside of the loop (we canonicalize away from this form, but it happens e.g. unanalyzeable loops) we'd avoid performing the uadd/usub transform.  Interestingly, all of these involve moving the increment closer to it's operands, so there's no concern about dominating all uses.  We can handle that case cheaply, resulting in a more robust transform.