[turbofan] Dump graph in RPO order as text.

src/compiler/graph-visualizer.h

Index: src/compiler/graph-visualizer.h
diff --git a/src/compiler/graph-visualizer.h b/src/compiler/graph-visualizer.h
index 7212a4f932d55689f6fc694f32604e83159c52e6..3dd66eaf41f3de71182e71902666f3d9f44f3339 100644
--- a/src/compiler/graph-visualizer.h
+++ b/src/compiler/graph-visualizer.h
@@ -36,6 +36,14 @@ struct AsJSON {
 
 std::ostream& operator<<(std::ostream& os, const AsJSON& ad);
 
+struct AsRPO {
+  explicit AsRPO(const Graph& g) : graph(g) {}
+  const Graph& graph;
+};
+
+std::ostream& operator<<(std::ostream& os, const AsRPO& ad);
+
+
 struct AsC1VCompilation {
   explicit AsC1VCompilation(const CompilationInfo* info) : info_(info) {}
   const CompilationInfo* info_;