[llvm] [AMDGPU] Promote uniform ops to i32 in GISel (PR #106557)

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

> Should add a MIR test that ensures this doesn't crash on a preset register class.
> 
> The codegen changes look worse to me, and I don't see any improvements. I think the issue is this is trying to promote after regbankselect, when most of the legalization up to scalar operations already occurred

Should we perhaps leave the CGP expansion in for the time being, and just disable it for DAGISel?
I'd also prefer to see this prelegalizer or in the legalizer.
I'm not sure how difficult it is to add UniformityInfo to a combiner pass, should I try that and see if I can move the combine in the PreLegalizer?

https://github.com/llvm/llvm-project/pull/106557