[X86] combineINSERT_SUBVECTOR - use concatSubVectors instead of direct fold to X86ISD::SUBV_BROADCAST_LOAD (#140919)

Use common helper and try to reduce the number of places we're
generating load node directly.

Author: RKSimon

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -59360,7 +59360,8 @@ static SDValue combineINSERT_SUBVECTOR(SDNode *N, SelectionDAG &DAG,
   }
 
   // If we're splatting the lower half subvector of a full vector load into the
-  // upper half, attempt to create a subvector broadcast.
+  // upper half, just splat the subvector directly, potentially creating a
+  // subvector broadcast.
   // TODO: Drop hasOneUse checks.
   if ((int)IdxVal == (VecNumElts / 2) &&
       Vec.getValueSizeInBits() == (2 * SubVec.getValueSizeInBits()) &&
@@ -59370,12 +59371,7 @@ static SDValue combineINSERT_SUBVECTOR(SDNode *N, SelectionDAG &DAG,
     if (VecLd && SubLd &&
         DAG.areNonVolatileConsecutiveLoads(
             SubLd, VecLd, SubVec.getValueSizeInBits() / 8, 0)) {
-      SDValue BcastLd = getBROADCAST_LOAD(X86ISD::SUBV_BROADCAST_LOAD, dl, OpVT,
-                                          SubVecVT, SubLd, 0, DAG);
-      SDValue NewSubVec = DAG.getNode(ISD::EXTRACT_SUBVECTOR, dl, SubVecVT,
-                                      BcastLd, DAG.getVectorIdxConstant(0, dl));
-      DCI.CombineTo(SubLd, NewSubVec, BcastLd.getValue(1));
-      return BcastLd;
+      return concatSubVectors(SubVec, SubVec, DAG, dl);
     }
   }