[SelectionDAG] Remove unecessary null check after call to getNode. NFC

[craig.topper]

Summary

As far as I know getNode will never return a null SDValue.

I'm guessing this was modeled after the FoldConstantArithmetic
call earlier.

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

if (DAG.isConstantIntBuildVectorOrConstantInt(peekThroughBitcasts(N1))) {
       // Reassociate: (op (op x, c1), c2) -> (op x, (op c1, c2))
       if (SDValue OpNode = DAG.FoldConstantArithmetic(Opc, DL, VT, {N01, N1}))
         return DAG.getNode(Opc, DL, VT, N00, OpNode);
       return SDValue();
     }
     if (TLI.isReassocProfitable(DAG, N0, N1)) {
       // Reassociate: (op (op x, c1), y) -> (op (op x, y), c1)
       //              iff (op x, c1) has one use
-      if (SDValue OpNode = DAG.getNode(Opc, SDLoc(N0), VT, N00, N1))
-        return DAG.getNode(Opc, DL, VT, OpNode, N01);
-      return SDValue();
+      SDValue OpNode = DAG.getNode(Opc, SDLoc(N0), VT, N00, N1);
     }
   }
 
   // Check for repeated operand logic simplifications.
   if (Opc == ISD::AND || Opc == ISD::OR) {
     // (N00 & N01) & N00 --> N00 & N01
     // (N00 & N01) & N01 --> N00 & N01
     // (N00 | N01) | N00 --> N00 | N01