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

[Thorsten Schütt via llvm-commits]

tschuett wrote:

I found 
https://github.com/llvm/llvm-project/blob/dac9042cc6a4dcb23e85b13232e2b233d55eda57/llvm/lib/CodeGen/GlobalISel/Utils.cpp#L405
which is also fallible and in an anonymous namespace.

The other option is to change the signature of the match functions:
```
bool CombinerHelper::matchFoldAPlusC1MinusC2(const MachineInstr &MI, const MachineInstr &C1, const MachineInstr &C2,
                                             BuildFnTy &MatchInfo) {
```
with the implication that C1 and C2 are G_CONSTANTs. I don't like because it is too low-level.

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