Potential null pointer access in LoadNode::split_through_phi after JDK-8287061

SonarCloud reports "base" can be null on this path after JDK-8287061:

```
  Node* phi = nullptr;
  const Type* this_type = this->bottom_type();
  PhaseIterGVN* igvn = phase->is_IterGVN();
  if (t_oop != nullptr && (t_oop->is_known_instance_field() || load_boxed_values)) {
    int this_index = C->get_alias_index(t_oop);
    int this_offset = t_oop->offset();
    int this_iid = t_oop->is_known_instance_field() ? t_oop->instance_id() : base->_idx; // <------ HERE
    phi = new PhiNode(region, this_type, nullptr, mem->_idx, this_iid, this_index, this_offset);
  } else if (ignore_missing_instance_id) {
    phi = new PhiNode(region, this_type, nullptr, mem->_idx);
  } else {
    return nullptr;
  }
```

This might be a false positive, but I don't clearly see what guarantees `base` not to be `nullptr` on that path. Previous code checks for `base_is_phi` (which does the nullcheck for `base`) before accessing `base` fields.

Is this a bug, Cesar? Feel free to close as "Not an Issue" if this is a false positive.