Just use insert_vector_elt

Author: arsenm

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -27533,18 +27533,11 @@ static SDValue scalarizeBinOpOfSplats(SDNode *N, SelectionDAG &DAG,
 
   // If all lanes but 1 are undefined, no need to splat the scalar result.
   // TODO: Keep track of undefs and use that info in the general case.
-  if (N0.getOpcode() == ISD::BUILD_VECTOR && N0.getOpcode() == N1.getOpcode() &&
-      // This is assuming if either input is undef, the result will fold out.
-      //
-      // TODO: Do we need to check if the opcode/operand propagates undef?
-      // Should we ignore operation identity values?
-      ((count_if(N0->ops(), [](SDValue V) { return !V.isUndef(); }) == 1) ||
-       (count_if(N1->ops(), [](SDValue V) { return !V.isUndef(); }) == 1))) {
+  if (N0.getOpcode() == ISD::BUILD_VECTOR && N0.getOpcode() == N1.getOpcode()) {
     // bo (build_vec ..undef, X, undef...), (build_vec ..undef, Y, undef...) -->
-    // build_vec ..undef, (bo X, Y), undef...
-    SmallVector<SDValue, 8> Ops(VT.getVectorNumElements(), DAG.getUNDEF(EltVT));
-    Ops[Index0] = ScalarBO;
-    return DAG.getBuildVector(VT, DL, Ops);
+    //   insert_vector_elt undef, (bo X, Y), index
+    return DAG.getNode(ISD::INSERT_VECTOR_ELT, DL, VT, DAG.getUNDEF(VT),
+                       ScalarBO, IndexC);
   }
 
   // bo (splat X, Index), (splat Y, Index) --> splat (bo X, Y), Index