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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I tried to detect these cases and revert the optimisation in amdgpu-codegenprepare, but realised that in some cases (e.g. when immediate can be encoded into instruction, depending on the target and the constant) it can increase registers usage (as well as instructions count).

That would be too late, plenty of compare and select patterns appear during legalization 


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