Shenandoah: Duplicated code in ShenandoahBarrierSetC2::ideal_node()

Node::set_req_X definition as:
void Node::set_req_X(uint i, Node *n, PhaseGVN *gvn) {
  PhaseIterGVN* igvn = gvn->is_IterGVN();
  if (igvn == NULL) {
    set_req(i, n);
    return;
  }
  set_req_X(i, n, igvn);
}

ShenandoahBarrierSetC2::ideal_node() duplicates above logic.