Refactor the code that checks that all operands of a node are UNDEFs.

Add a micro-optimization to getNode of CONCAT_VECTORS when both operands are undefs.
Can't find a testcase for this because VECTOR_SHUFFLE already handles undef operands, but Duncan suggested that we add this.

Together with Michael Kuperstein <michael.m.kuperstein@intel.com>



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@160229 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/SelectionDAGNodes.h

@@ -74,6 +74,10 @@ namespace ISD {
   /// ISD::SCALAR_TO_VECTOR node or a BUILD_VECTOR node where only the low
   /// element is not an undef.
   bool isScalarToVector(const SDNode *N);
+
+  /// allOperandsUndef - Return true if the node has at least one operand
+  /// and all operands of the specified node are ISD::UNDEF.
+  bool allOperandsUndef(const SDNode *N);
 }  // end llvm:ISD namespace
 
 //===----------------------------------------------------------------------===//