[CodeGen] This patch fix a bug that may caused error for a self-defined target...

[CodeGen] This patch fix a bug that may caused error for a self-defined target in SelectionDAG::getNode (#75320)

we need first judge N1.getNumOperands() > 0.

If Lowering Generated SDNode like.

```
v2i32 t20:  TargetOpNode.
i32 t21: extract_vector_elt t20  0
i32 t22: extract_vector_elt t20 1
```

will cause a error.