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

[Luke Lau via llvm-commits]

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@@ -25173,6 +25173,38 @@ static SDValue combineConcatVectorOfCasts(SDNode *N, SelectionDAG &DAG) {
   return DAG.getNode(CastOpcode, DL, VT, NewConcat);
 }
 
+static SDValue combineConcatVectorInterleave(SDNode *N, SelectionDAG &DAG) {
+  SDValue FirstOp = N->getOperand(0);
+  if (FirstOp.getOpcode() != ISD::VECTOR_INTERLEAVE ||
+      FirstOp.getNumOperands() != N->getNumOperands())
+    return SDValue();
+
+  for (unsigned I = 0; I < N->getNumOperands(); I++) {
+    if (N->getOperand(I).getResNo() != I ||
+        N->getOperand(I).getNode() != FirstOp.getNode())
+      return SDValue();
+  }
+
+  SDValue InOp0 = FirstOp.getOperand(0);
+  if (!llvm::all_of(FirstOp->ops(),
+                    [&InOp0](SDValue Op) { return Op == InOp0; }))
+    return SDValue();
+
+  // We're concatenating all the sequential results of the same vector
+  // interleave node. Now check if all inputs to the interleave are splats.
+  if (SDValue Splat = DAG.getSplatValue(InOp0)) {
----------------
lukel97 wrote:

Nit, early return instead?

```suggestion
  SDValue Splat = DAG.getSplatValue(InOp0);
  if (!Splat)
    return SDValue();
```

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