Remove operator<< from HGraph.

It needs access to a code generator.

Test: observe a bogus graph at the point of register allocator, and see
that the Dump method crashes.

Change-Id: I09a847246428fd3bd0938f9f457773c465da253d
diff --git a/compiler/optimizing/nodes.cc b/compiler/optimizing/nodes.cc
index 2478693..00bfc92 100644
--- a/compiler/optimizing/nodes.cc
+++ b/compiler/optimizing/nodes.cc
@@ -2014,8 +2014,9 @@
 }
 
 std::ostream& HGraph::Dump(std::ostream& os,
+                           CodeGenerator* codegen,
                            std::optional<std::reference_wrapper<const BlockNamer>> namer) {
-  HGraphVisualizer vis(&os, this, nullptr, namer);
+  HGraphVisualizer vis(&os, this, codegen, namer);
   vis.DumpGraphDebug();
   return os;
 }