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

[Matt Arsenault via llvm-commits]

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@@ -2001,28 +2001,6 @@ SDValue NVPTXTargetLowering::LowerSTACKSAVE(SDValue Op,
   return DAG.getMergeValues({ASC, SDValue(SS.getNode(), 1)}, DL);
 }
 
-// By default CONCAT_VECTORS is lowered by ExpandVectorBuildThroughStack()
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arsenm wrote:

So this was previously dead code? 

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