ValueTracking: Fix isKnownNonZero for non-0 null pointers for byval

The IR doesn't have a proper concept of invalid pointers, and "null"
constants are just all zeros (though it really needs one).

I think it's not possible to break this for AMDGPU due to the copy
semantics of byval. If you have an original stack object at 0, the
byval copy will be placed above it so I don't think it's really
possible to hit a 0 address.