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

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

@arsenm Jay found a way to make the pattern in #69703 work so this in not needed to fix #59869

Should I abandon this, or is this still a worthwhile addition? There's some positive code changes but not many tests are affected so not sure it's worth it. OTOH the code's already done so it doesn't hurt to just land this too.
No strong opinion.

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