[llvm] [NFC] Make AMDGPUCombinerHelper methods const (PR #121903)

[Paul Bowen-Huggett via llvm-commits]

paulhuggett wrote:

Thanks @arsenm. I pushed an extra commit after you approved merge to actually eliminate the "TODO" that was my goal! Hope that's okay…

I don’t have commit access; if you're happy, can you merge this PR for me?

https://github.com/llvm/llvm-project/pull/121903