DAG: Use poison when legalizing scalar_to_vector results (#167751)

Author: arsenm

llvm/include/llvm/CodeGen/ISDOpcodes.h

@@ -655,8 +655,8 @@ enum NodeType {
 
   /// SCALAR_TO_VECTOR(VAL) - This represents the operation of loading a
   /// scalar value into element 0 of the resultant vector type.  The top
-  /// elements 1 to N-1 of the N-element vector are undefined.  The type
-  /// of the operand must match the vector element type, except when they
+  /// elements 1 to N-1 of the N-element vector are poison. The type of
+  /// the operand must match the vector element type, except when they
   /// are integer types.  In this case the operand is allowed to be wider
   /// than the vector element type, and is implicitly truncated to it.
   SCALAR_TO_VECTOR,

llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp

@@ -2186,7 +2186,7 @@ void DAGTypeLegalizer::SplitVecRes_ScalarOp(SDNode *N, SDValue &Lo,
   std::tie(LoVT, HiVT) = DAG.GetSplitDestVTs(N->getValueType(0));
   Lo = DAG.getNode(N->getOpcode(), dl, LoVT, N->getOperand(0));
   if (N->getOpcode() == ISD::SCALAR_TO_VECTOR) {
-    Hi = DAG.getUNDEF(HiVT);
+    Hi = DAG.getPOISON(HiVT);
   } else {
     assert(N->getOpcode() == ISD::SPLAT_VECTOR && "Unexpected opcode");
     Hi = Lo;