SelectionDAG: Add missing AddNodeIDCustom case for MDNodeSDNode.

Without this we ended up never deduplicating MDNodeSDNodes.

Reviewers: arsenm

Reviewed By: arsenm

Pull Request: llvm/llvm-project#136805

Author: pcc

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -967,6 +967,9 @@ static void AddNodeIDCustom(FoldingSetNodeID &ID, const SDNode *N) {
   case ISD::INTRINSIC_W_CHAIN:
     // Handled by MemIntrinsicSDNode check after the switch.
     break;
+  case ISD::MDNODE_SDNODE:
+    ID.AddPointer(cast<MDNodeSDNode>(N)->getMD());
+    break;
   } // end switch (N->getOpcode())
 
   // MemIntrinsic nodes could also have subclass data, address spaces, and flags