[NFC] Improve comment about inverse depth-first iterator + clang-format

[bmahjour]

Summary

Stumbled upon this comment in GraphTraits.h and found it a bit hard to understand. I think it would be more useful to define the equivalent of an idf_iterator entirely based on a df_iterator. Agree?

I also ran clang-format.

llvm/include/llvm/ADT/GraphTraits.h

 //
 // This template evolved from supporting `BasicBlock` to also later supporting
 // more complex types (e.g. CFG and DomTree).
 //
 // GraphTraits can be used to create a view over a graph interpreting it
 // differently without requiring a copy of the original graph. This could
 // be achieved by carrying more data in NodeRef. See LoopBodyTraits for one
 // example.
-template<class GraphType>
-struct GraphTraits {
+template <class GraphType> struct GraphTraits {
   // Elements to provide:
 
   // typedef NodeRef           - Type of Node token in the graph, which should
   //                             be cheap to copy.
   // typedef ChildIteratorType - Type used to iterate over children in graph,
   //                             dereference to a NodeRef.
 
   // static NodeRef getEntryNode(const GraphType &)

 // Not all graphs define an inverse ordering, and if they do, it depends on
 // the graph exactly what that is.  Here's an example of usage with the
 // df_iterator:
 //
 // idf_iterator<Method*> I = idf_begin(M), E = idf_end(M);
 // for (; I != E; ++I) { ... }
 //
 // Which is equivalent to:
-// df_iterator<Inverse<Method*>> I = idf_begin(M), E = idf_end(M);
+// df_iterator<Inverse<Method *>> I = df_begin(Inverse<Method *>(M)),
+//                                E = df_end(Inverse<Method *>(M));
 // for (; I != E; ++I) { ... }
 //
-template <class GraphType>
-struct Inverse {
+template <class GraphType> struct Inverse {
   const GraphType &Graph;
 
   inline Inverse(const GraphType &G) : Graph(G) {}
 };
 
 // Provide a partial specialization of GraphTraits so that the inverse of an
 // inverse falls back to the original graph.
 template <class T> struct GraphTraits<Inverse<Inverse<T>>> : GraphTraits<T> {};