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

[Ethan Kaji via llvm-commits]

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@@ -3371,7 +3392,7 @@ bool SelectionDAGLegalize::ExpandNode(SDNode *Node) {
     Results.push_back(ExpandInsertToVectorThroughStack(SDValue(Node, 0)));
     break;
   case ISD::CONCAT_VECTORS:
-    Results.push_back(ExpandVectorBuildThroughStack(Node));
+    Results.push_back(ExpandConcatVectors(Node));
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Esan5 wrote:

Unfortunately it doesn't seem like `isOperationLegalOrCustom` works. I might be wrong, but in `Target/AArch64/AArch64TargetLowering.cpp`, it looks like `EXTRACT_VECTOR_ELT` is either legal or custom. Would it be possible to make the necessary change there?

Making the check `TLI.isOperationLegal` also doesn't change the test case arsenm brought up.

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