Reuse backing store of two zone lists by using Rewind(0) instead of Clear().

src/hydrogen.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 361 matching lines @@
   HEnvironment* CopyForInlining(Handle<JSFunction> target,
                                 FunctionLiteral* function,
                                 bool is_speculative,
                                 HConstant* undefined) const;
 
   void AddIncomingEdge(HBasicBlock* block, HEnvironment* other);
 
   void ClearHistory() {
     pop_count_ = 0;
     push_count_ = 0;
-    assigned_variables_.Clear();
+    assigned_variables_.Rewind(0);
   }
 
   void SetValueAt(int index, HValue* value) {
     ASSERT(index < length());
     values_[index] = value;
   }
 
   void PrintTo(StringStream* stream);
   void PrintToStd();
 
@@ skipping 698 matching lines @@
   const char* filename_;
   HeapStringAllocator string_allocator_;
   StringStream trace_;
   int indent_;
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_H_