[llvm] [GlobalIsel] Combine G_ADD and G_SUB with constants (PR #97771)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

I can remove the comments. The alternative would be to pass the `G_CONSTANT` s as MachineInstr to the match function and then manually extract the immediate, but this is really ugly.

https://github.com/llvm/llvm-project/pull/97771