[llvm] [AMDGPU] Fix computed kill mask (PR #122736)

[Jay Foad via llvm-commits]

https://github.com/jayfoad commented:

Seems plausible but I don't fully understand how this instruction fits into the whole `lowerKillI1` code sequence. Is there a comment somewhere that explains the whole code sequence?

https://github.com/llvm/llvm-project/pull/122736