[strong] Simplify (and sortof optimize) string addition for strong mode.

src/builtins.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_BUILTINS_H_
 #define V8_BUILTINS_H_
 
 namespace v8 {
 namespace internal {
 
@@ skipping 168 matching lines @@
   V(IN, 1)                                 \
   V(INSTANCE_OF, 1)                        \
   V(CALL_NON_FUNCTION, 0)                  \
   V(CALL_NON_FUNCTION_AS_CONSTRUCTOR, 0)   \
   V(CALL_FUNCTION_PROXY, 1)                \
   V(CALL_FUNCTION_PROXY_AS_CONSTRUCTOR, 1) \
   V(TO_NUMBER, 0)                          \
   V(TO_STRING, 0)                          \
   V(TO_NAME, 0)                            \
   V(STRING_ADD_LEFT, 1)                    \
-  V(STRING_ADD_LEFT_STRONG, 1)             \
   V(STRING_ADD_RIGHT, 1)                   \
-  V(STRING_ADD_RIGHT_STRONG, 1)            \
   V(APPLY_PREPARE, 1)                      \
   V(REFLECT_APPLY_PREPARE, 1)              \
   V(REFLECT_CONSTRUCT_PREPARE, 2)          \
   V(CONCAT_ITERABLE_TO_ARRAY, 1)           \
   V(STACK_OVERFLOW, 1)
 
 class BuiltinFunctionTable;
 class ObjectVisitor;
 
 
@@ skipping 146 matching lines @@
 
   friend class BuiltinFunctionTable;
   friend class Isolate;
 
   DISALLOW_COPY_AND_ASSIGN(Builtins);
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_BUILTINS_H_