[SCEV] Handle unreachable binop when matching shift recurrence

[SCEV] Handle unreachable binop when matching shift recurrence

This fixes an issue introduced with my change d4648e, and reported in pr49768.

The root problem is that dominance collapses in unreachable code, and that LoopInfo explicitly only models reachable code.  Since the recurrence matcher doesn't filter by reachability (and can't easily because not all consumers have domtree), we need to bailout before assuming that finding a recurrence implies we found a loop.