diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 45967372d421..73b90666c568 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -15350,6 +15350,8 @@ static SDValue simplifyShuffleMask(ShuffleVectorSDNode *SVN, SDValue N0,
     // TODO - handle more cases as required.
     if (V.getOpcode() == ISD::BUILD_VECTOR)
       return V.getOperand(Idx).isUndef();
+    if (V.getOpcode() == ISD::SCALAR_TO_VECTOR)
+      return (Idx != 0) || V.getOperand(0).isUndef();
     return false;
   };
 
@@ -15463,6 +15465,7 @@ static SDValue combineShuffleOfScalars(ShuffleVectorSDNode *SVN,
 
   if (!N0->hasOneUse() || !N1->hasOneUse())
     return SDValue();
+
   // If only one of N1,N2 is constant, bail out if it is not ALL_ZEROS as
   // discussed above.
   if (!N1.isUndef()) {
@@ -15484,8 +15487,8 @@ static SDValue combineShuffleOfScalars(ShuffleVectorSDNode *SVN,
       if (S.getOpcode() == ISD::BUILD_VECTOR) {
         Op = S.getOperand(Idx);
       } else if (S.getOpcode() == ISD::SCALAR_TO_VECTOR) {
-        if (Idx == 0)
-          Op = S.getOperand(0);
+        assert(Idx == 0 && "Unexpected SCALAR_TO_VECTOR operand index.");
+        Op = S.getOperand(0);
       } else {
         // Operand can't be combined - bail out.
         return SDValue();
@@ -15501,6 +15504,7 @@ static SDValue combineShuffleOfScalars(ShuffleVectorSDNode *SVN,
 
     Ops.push_back(Op);
   }
+
   // BUILD_VECTOR requires all inputs to be of the same type, find the
   // maximum type and extend them all.
   EVT SVT = VT.getScalarType();