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

src/hydrogen.h

Index: src/hydrogen.h
diff --git a/src/hydrogen.h b/src/hydrogen.h
index 9aac71c93f37ac2671286b2d384b0ed807d857bb..60a4420024d3ad4019de686e7e880ecd2a6fd28e 100644
--- a/src/hydrogen.h
+++ b/src/hydrogen.h
@@ -976,9 +976,8 @@ class HGraphBuilder: public AstVisitor {
                        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);