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

[Justin Fargnoli via llvm-commits]

================
@@ -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));
----------------
justinfargnoli wrote:

My guess is that we'd want to do something like:
```suggestion
    if (TLI.isLegalOrCustom(ISD::EXTRACT_VECTOR_ELT, ElementValueType)
        Results.push_back(ExpandConcatVectors(Node));
    else
        Results.push_back(ExpandVectorBuildThroughStack(Node));
```

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