[InstCombine] Generalize icmp handling in isKnownNonZero()

The dominating condition handling in isKnownNonZero() currently
only takes into account conditions of the form "x != 0" or "x == 0".
However, there are plenty of other conditions that imply non-zero,
a common one being "x s> 0".

Peculiarly, the handling for assumes was already dealing with more
general non-zero-ness conditions, so this just reuses the same
logic for the dominating condition case.