[X86] collectConcatOps - add ability to recurse through insert_subvector chains

Allows us to match insert_subvector(insert_subvector(undef, insert_subvector(insert_subvector(undef, x, 0), y, 1), 0), 0),
                                    insert_subvector(insert_subvector(undef, z, 0), w, 1), 2)

Author: RKSimon

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -3992,7 +3992,6 @@ static bool collectConcatOps(SDNode *N, SmallVectorImpl<SDValue> &Ops,
     EVT VT = Src.getValueType();
     EVT SubVT = Sub.getValueType();
 
-    // TODO - Handle more general insert_subvector chains.
     if (VT.getSizeInBits() == (SubVT.getSizeInBits() * 2)) {
       // insert_subvector(undef, x, lo)
       if (Idx == 0 && Src.isUndef()) {
@@ -4005,8 +4004,19 @@ static bool collectConcatOps(SDNode *N, SmallVectorImpl<SDValue> &Ops,
         if (Src.getOpcode() == ISD::INSERT_SUBVECTOR &&
             Src.getOperand(1).getValueType() == SubVT &&
             isNullConstant(Src.getOperand(2))) {
-          Ops.push_back(Src.getOperand(1));
-          Ops.push_back(Sub);
+          // Attempt to recurse into inner (matching) concats.
+          SDValue Lo = Src.getOperand(1);
+          SDValue Hi = Sub;
+          SmallVector<SDValue, 2> LoOps, HiOps;
+          if (collectConcatOps(Lo.getNode(), LoOps, DAG) &&
+              collectConcatOps(Hi.getNode(), HiOps, DAG) &&
+              LoOps.size() == HiOps.size()) {
+            Ops.append(LoOps);
+            Ops.append(HiOps);
+            return true;
+          }
+          Ops.push_back(Lo);
+          Ops.push_back(Hi);
           return true;
         }
         // insert_subvector(x, extract_subvector(x, lo), hi)