[SelectionDAG] remove redundant code; NFCI

getNode() squashes concatenation of undefs via FoldCONCAT_VECTORS():
  // Concat of UNDEFs is UNDEF.
  if (llvm::all_of(Ops, [](SDValue Op) { return Op.isUndef(); }))
    return DAG.getUNDEF(VT);

llvm-svn: 361284