[AMDGPU] Fix computed kill mask (#122736) Replace S_XOR with S_ANDN2 when computing the kill mask in demote/kill lowering. This has the effect of AND'ing demote/kill condition with exec which is needed for proper live mask update. The S_XOR is inadequate because it may return true for lane with exec=0. This patch fixes an image corruption in game. I think the issue went unnoticed because demote/kill condition is often naturally dependent on exec, so AND'ing with exec is usually not required.
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