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

[Jay Foad via llvm-commits]

jayfoad wrote:

> > 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.
> 
> Is it improving in cross block cases? Add a comment that it should be removable after globalisel?

My preference is still not to add this complexity to SIFoldOperands. Are there still "positive code changes" if you rebase this on #69703?

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