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

[Carl Ritson via llvm-commits]

perlfu wrote:

> 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?

I don’t think there is a comment covering the whole sequence.
This branch is specific to when a condition passed to kill or demote represents the inverse of the lanes to be killed.
It is responsible for preparing the mask to be applied to the shader maintained set of live lanes (which is a subset of exec).
The result will update the live lanes which will then be used to update exec.

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