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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Are these docs public? I had a hard time finding information on what these intrinsics do.

Mostly yes. https://llvm.org/docs/AMDGPUUsage.html#additional-documentation links to the various ISA public documents (see the MIMG sections for dmask). The intrinsics for the most part directly map to the instructions of the same names. It covers dmask but sometimes unusual edge cases aren't mentioned in the public docs (like I don't see the 0 case mentioned here)

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