[Attributor][NFC] Update description for the dependency graph

The word "dependency graph" is a bit misleading. When there is an
edge from node A to B (A -> B), it actually mean that B depends on
A and when the state of A is updated, B should also be updated. So
I update the comment to make the description clearer.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D85065

llvm/include/llvm/Transforms/IPO/Attributor.h

@@ -155,8 +155,8 @@ struct AADepGraphNode {
   using DepTy = PointerIntPair<AADepGraphNode *, 1>;
 
 protected:
-  /// Set of dependency graph nodes which this one depends on.
-  /// The bit encodes if it is optional.
+  /// Set of dependency graph nodes which should be updated if this one
+  /// is updated. The bit encodes if it is optional.
   TinyPtrVector<DepTy> Deps;
 
   static AADepGraphNode *DepGetVal(DepTy &DT) { return DT.getPointer(); }
@@ -184,6 +184,11 @@ struct AADepGraphNode {
   friend struct AADepGraph;
 };
 
+/// The data structure for the dependency graph
+///
+/// Note that in this graph if there is an edge from A to B (A -> B),
+/// then it means that B depends on A, and when the state of A is
+/// updated, node B should also be updated
 struct AADepGraph {
   AADepGraph() {}
   ~AADepGraph() {}
@@ -197,7 +202,6 @@ struct AADepGraph {
   /// requires a single entry point, so we maintain a fake("synthetic") root
   /// node that depends on every node.
   AADepGraphNode SyntheticRoot;
-
   AADepGraphNode *GetEntryNode() { return &SyntheticRoot; }
 
   iterator begin() { return SyntheticRoot.child_begin(); }