[llvm] [AMDGPU] Try to reuse in v_cndmask register with constant from compare. (PR #131146)

[Jay Foad via llvm-commits]

jayfoad wrote:

Is this going to match `X == +0.0 ? X : Y` --> `X == +0.0 ? +0.0 : Y`? That would be wrong, because if X is -0.0 then the first one evaluates to -0.0 and the second one evaluates to +0.0.

https://github.com/llvm/llvm-project/pull/131146