Changes

[InstCombine] Add additional tests for known non zero (NFC)

Check conditions that imply non-zero, even if they are not literally
"x != 0".

Using ctlz for testing, as explicit comparison might get folded by
other reasoning.

[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.