Maintain the invariant that all HGraphBuilder::VisitFoo methods return void.

src/hydrogen.h

 // Copyright 2012 the V8 project authors. All rights reserved.
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 // 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
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@@ skipping 958 matching lines @@
   // contexts.  (It's allowed in all effect contexts and disallowed in all
   // test contexts.)
   void VisitForValue(Expression* expr,
                      ArgumentsAllowedFlag flag = ARGUMENTS_NOT_ALLOWED);
   void VisitForTypeOf(Expression* expr);
   void VisitForEffect(Expression* expr);
   void VisitForControl(Expression* expr,
                        HBasicBlock* true_block,
                        HBasicBlock* false_block);
 
-  // Visit an argument subexpression and emit a push to the outgoing
-  // arguments.  Returns the hydrogen value that was pushed.
-  HValue* VisitArgument(Expression* expr);
+  // Visit an argument subexpression and emit a push to the outgoing arguments.
+  void VisitArgument(Expression* expr);
 
   void VisitArgumentList(ZoneList<Expression*>* arguments);
 
   // Visit a list of expressions from left to right, each in a value context.
   void VisitExpressions(ZoneList<Expression*>* exprs);
 
   void AddPhi(HPhi* phi);
 
   void PushAndAdd(HInstruction* instr);
 
@@ skipping 459 matching lines @@
   const char* filename_;
   HeapStringAllocator string_allocator_;
   StringStream trace_;
   int indent_;
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_H_