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

[Thorsten Schütt via llvm-commits]

tschuett wrote:

I am open for other names, but I like the pattern: `APInt C2 = getIConstantFromReg(Sub->getRHSReg(), MRI);`.

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