Refactor Hydrogen environment liveness analysis into an HPhase.

src/hydrogen.h

Index: src/hydrogen.h
diff --git a/src/hydrogen.h b/src/hydrogen.h
index 6a49e5708de953eb5cb74e88906afe9526c18b79..63f38ce917ea6ee80d11ec362c4bdf4fd119eaf4 100644
--- a/src/hydrogen.h
+++ b/src/hydrogen.h
@@ -428,6 +428,8 @@ class HGraph: public ZoneObject {
   HConstant* GetConstant(SetOncePointer<HConstant>* pointer,
                          int32_t integer_value);
 
+  template<typename Phase>
+  void Run() { Phase phase(this, info()->phase_zone()); phase.Run(); }
   void MarkLive(HValue* ref, HValue* instr, ZoneList<HValue*>* worklist);
   void MarkLiveInstructions();
   void RemoveDeadInstructions();