[llvm] Port `NVPTXTargetLowering::LowerCONCAT_VECTORS` to SelectionDAG (PR #120030)

[Matt Arsenault via llvm-commits]

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@@ -1517,10 +1518,28 @@ SDValue SelectionDAGLegalize::ExpandInsertToVectorThroughStack(SDValue Op) {
                      BaseVecAlignment);
 }
 
+SDValue SelectionDAGLegalize::ExpandConcatVectors(SDNode *Node) {
+  assert(Node->getOpcode() == ISD::CONCAT_VECTORS && "Unexpected opcode!");
+  SDLoc Dl(Node);
+  SmallVector<SDValue, 0> Ops;
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arsenm wrote:

```suggestion
  SmallVector<SDValue, 16> Ops;
```

https://github.com/llvm/llvm-project/pull/120030