[SelectionDAGISel] Add back a comment to MergeInputChains handling. NFC

This comment used to exist, but was lost in a refactor over 10 years
ago, but still seems relevant and improves readability.

Author: topperc

llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp

@@ -3269,6 +3269,8 @@ void SelectionDAGISel::SelectCodeCommon(SDNode *NodeToMatch,
       assert(RecNo < RecordedNodes.size() && "Invalid EmitMergeInputChains");
       ChainNodesMatched.push_back(RecordedNodes[RecNo].first.getNode());
 
+      // If the chained node is not the root, we can't fold it if it has
+      // multiple uses.
       // FIXME: What if other value results of the node have uses not matched
       // by this pattern?
       if (ChainNodesMatched.back() != NodeToMatch &&
@@ -3306,6 +3308,8 @@ void SelectionDAGISel::SelectCodeCommon(SDNode *NodeToMatch,
         assert(RecNo < RecordedNodes.size() && "Invalid EmitMergeInputChains");
         ChainNodesMatched.push_back(RecordedNodes[RecNo].first.getNode());
 
+        // If the chained node is not the root, we can't fold it if it has
+        // multiple uses.
         // FIXME: What if other value results of the node have uses not matched
         // by this pattern?
         if (ChainNodesMatched.back() != NodeToMatch &&