[AMDGPU] Use poison instead of undef for non-demanded elements (#75914)

Return poison instead of undef for non-demanded lanes in the AMDGPU
demanded element simplification hook.

Also bail out of dmask is 0, as this case has special semantics:

> If DMASK==0, the TA overrides DMASK=1 and puts zeros in VGPR followed by
> LWE status if exists. TFE status is not generated since the fetch is dropped.