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

[Matt Arsenault via llvm-commits]

https://github.com/arsenm approved this pull request.

There shouldn't be a reason to prefer undef over poison here. However I'm wondering if it's correct to use undef in the first place. I briefly skimmed the hardware docs for images and I believe the behavior for 0 is to actually act like dmask=1 and return a 0

https://github.com/llvm/llvm-project/pull/75914