[llvm] [GlobalIsel] Combine G_SELECT (PR #74845)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

Regarding the AMDGPU test changes. I played with it. During legalization G_SELECTs are introduced. The combiner is now more aggressive to combine them. The changes ought to fall into the optimization category rather then random churn. 

https://github.com/llvm/llvm-project/pull/74845