[llvm] [DAGCombiner] Add combine for vector interleave of splats (PR #151110)

[Paul Walker via llvm-commits]

================
@@ -25274,6 +25281,32 @@ static SDValue combineConcatVectorOfShuffleAndItsOperands(
   return DAG.getVectorShuffle(VT, dl, ShufOps[0], ShufOps[1], Mask);
 }
 
+static SDValue combineConcatVectorOfSplats(SDNode *N, SelectionDAG &DAG,
+                                           const TargetLowering &TLI,
+                                           bool LegalTypes,
+                                           bool LegalOperations) {
+  EVT VT = N->getValueType(0);
+
+  // Post-legalization we can only create wider SPLAT_VECTOR operations if both
+  // the type and operation is legal. The Hexagon target has custom
+  // legalization for SPLAT_VECTOR that splits the operation into two parts and
+  // concatenates them. Therefore, custom lowering must also be rejected in
+  // order to avoid an infinite loop.
+  if ((LegalTypes && !TLI.isTypeLegal(VT)) ||
+      (LegalOperations && !TLI.isOperationLegal(ISD::SPLAT_VECTOR, VT)))
+    return SDValue();
+
+  if (!llvm::all_equal(N->op_values()) ||
+      N->getOperand(0).getOpcode() != ISD::SPLAT_VECTOR)
+    return SDValue();
+
+  SDValue Splat = DAG.getSplatValue(N->getOperand(0));
+  if (!Splat)
+    return SDValue();
+
+  return DAG.getNode(ISD::SPLAT_VECTOR, SDLoc(N), VT, Splat);
----------------
paulwalker-arm wrote:

Given `N->getOperand(0)` must be a SPLAT_VECTOR.
```suggestion
  return DAG.getNode(ISD::SPLAT_VECTOR, SDLoc(N), VT, N->getOperand(0).getOperand(0));
```

https://github.com/llvm/llvm-project/pull/151110