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

[Thorsten Schütt via llvm-commits]

tschuett wrote:

Thanks for chiming in. Surprise, I like `getIConstantVRegVal`. It goes through MRI and has to be fallible. The other option, I could see that tablegen creates a struct/class with the reconstructed pattern:
`bool matchFoo(const FooPattern *Pattern, BuildFnTy &MatchInfo);`

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