Rework BuildCheckString() to be compatible with the other BuildCheck*() methods.

src/code-stubs-hydrogen.cc

 // Copyright 2012 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 974 matching lines @@
 HValue* CodeStubGraphBuilder<NewStringAddStub>::BuildCodeInitializedStub() {
   NewStringAddStub* stub = casted_stub();
   StringAddFlags flags = stub->flags();
   PretenureFlag pretenure_flag = stub->pretenure_flag();
 
   HValue* left = GetParameter(NewStringAddStub::kLeft);
   HValue* right = GetParameter(NewStringAddStub::kRight);
 
   // Make sure that both arguments are strings if not known in advance.
   if ((flags & STRING_ADD_CHECK_LEFT) == STRING_ADD_CHECK_LEFT) {
-    IfBuilder if_leftnotstring(this);
-    if_leftnotstring.IfNot<HIsStringAndBranch>(left);
-    if_leftnotstring.Then();
-    if_leftnotstring.Deopt("Expected string for LHS of string addition");
+    left = BuildCheckString(left);
   }
   if ((flags & STRING_ADD_CHECK_RIGHT) == STRING_ADD_CHECK_RIGHT) {
-    IfBuilder if_rightnotstring(this);
-    if_rightnotstring.IfNot<HIsStringAndBranch>(right);
-    if_rightnotstring.Then();
-    if_rightnotstring.Deopt("Expected string for RHS of string addition");
+    right = BuildCheckString(right);
   }
 
   return BuildStringAdd(left, right, pretenure_flag);
 }
 
 
 Handle<Code> NewStringAddStub::GenerateCode(Isolate* isolate) {
   return DoGenerateCode(isolate, this);
 }
 
@@ skipping 298 matching lines @@
   return BuildUncheckedDictionaryElementLoad(receiver, key);
 }
 
 
 Handle<Code> KeyedLoadDictionaryElementStub::GenerateCode(Isolate* isolate) {
   return DoGenerateCode(isolate, this);
 }
 
 
 } }  // namespace v8::internal