[stubs] Port StringAddStub to TF

src/compiler/code-assembler.h

 // Copyright 2015 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_COMPILER_CODE_ASSEMBLER_H_
 #define V8_COMPILER_CODE_ASSEMBLER_H_
 
 #include <map>
 #include <memory>
 
@@ skipping 211 matching lines @@
   Node* NumberConstant(double value);
   Node* SmiConstant(Smi* value);
   Node* HeapConstant(Handle<HeapObject> object);
   Node* BooleanConstant(bool value);
   Node* ExternalConstant(ExternalReference address);
   Node* Float64Constant(double value);
   Node* NaNConstant();
 
   bool ToInt32Constant(Node* node, int32_t& out_value);
   bool ToInt64Constant(Node* node, int64_t& out_value);
+  bool ToSmiConstant(Node* node, Smi*& out_value);
   bool ToIntPtrConstant(Node* node, intptr_t& out_value);
 
   Node* Parameter(int value);
   void Return(Node* value);
 
   void DebugBreak();
   void Comment(const char* format, ...);
 
   void Bind(Label* label);
   void Goto(Label* label);
@@ skipping 269 matching lines @@
   // Map of variables to the list of value nodes that have been added from each
   // merge path in their order of merging.
   std::map<Variable::Impl*, std::vector<Node*>> variable_merges_;
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_CODE_ASSEMBLER_H_