[llvm] [AMDGPU] Fold uniform readfirstlane + cndmask (PR #70188)

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

It fixed the cases that the ticket wanted, this fixes other cases but it's too much effort for little benefit I think. I can always come back to this later if needed

https://github.com/llvm/llvm-project/pull/70188