[turbofan] Implement JSBuiltinReducer for String.prototype[Symbol.iterator].

src/compiler/js-builtin-reducer.h

 // Copyright 2014 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_JS_BUILTIN_REDUCER_H_
 #define V8_COMPILER_JS_BUILTIN_REDUCER_H_
 
 #include "src/base/compiler-specific.h"
 #include "src/base/flags.h"
 #include "src/compiler/graph-reducer.h"
@@ skipping 72 matching lines @@
   Reduction ReduceMathTanh(Node* node);
   Reduction ReduceMathTrunc(Node* node);
   Reduction ReduceNumberIsFinite(Node* node);
   Reduction ReduceNumberIsInteger(Node* node);
   Reduction ReduceNumberIsNaN(Node* node);
   Reduction ReduceNumberIsSafeInteger(Node* node);
   Reduction ReduceNumberParseInt(Node* node);
   Reduction ReduceStringCharAt(Node* node);
   Reduction ReduceStringCharCodeAt(Node* node);
   Reduction ReduceStringFromCharCode(Node* node);
+  Reduction ReduceStringIterator(Node* node);
   Reduction ReduceStringIteratorNext(Node* node);
   Reduction ReduceArrayBufferViewAccessor(Node* node,
                                           InstanceType instance_type,
                                           FieldAccess const& access);
 
   Node* ToNumber(Node* value);
   Node* ToUint32(Node* value);
 
   Flags flags() const { return flags_; }
   Graph* graph() const;
@@ skipping 11 matching lines @@
   TypeCache const& type_cache_;
 };
 
 DEFINE_OPERATORS_FOR_FLAGS(JSBuiltinReducer::Flags)
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_JS_BUILTIN_REDUCER_H_