[GlobalISel] Combine: (G_*MULO x, 0) -> 0 + no carry out

Similar to the following combine in `DAGCombiner::visitMULO`:

```
  // fold (mulo x, 0) -> 0 + no carry out
  if (isNullOrNullSplat(N1))
    return CombineTo(N, DAG.getConstant(0, DL, VT),
                     DAG.getConstant(0, DL, CarryVT));
```

This fixes some generally poor codegen for `*mulo`:

https://godbolt.org/z/eTxYsvz8f

Differential Revision: https://reviews.llvm.org/D118635