[turbofan] Introduce dedicated StringIndexOf operator.

src/builtins/builtins-string.cc

 // Copyright 2016 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.
 
 #include "src/builtins/builtins-utils.h"
 #include "src/builtins/builtins.h"
 #include "src/code-factory.h"
 #include "src/code-stub-assembler.h"
 #include "src/regexp/regexp-utils.h"
 
@@ skipping 846 matching lines @@
 }
 
 // ES6 String.prototype.indexOf(searchString [, position])
 // #sec-string.prototype.indexof
 // Unchecked helper for builtins lowering.
 TF_BUILTIN(StringIndexOf, StringBuiltinsAssembler) {
   Node* receiver = Parameter(0);
   Node* search_string = Parameter(1);
   Node* position = Parameter(2);
 
-  Label call_runtime(this);
-
   Node* instance_type = LoadInstanceType(receiver);
   Node* search_string_instance_type = LoadInstanceType(search_string);
 
   StringIndexOf(receiver, instance_type, search_string,
                 search_string_instance_type, position,
                 [this](Node* result) { this->Return(result); });
 }
 
 // ES6 String.prototype.indexOf(searchString [, position])
 // #sec-string.prototype.indexof
@@ skipping 618 matching lines @@
         CallRuntime(Runtime::kThrowIncompatibleMethodReceiver, context,
                     HeapConstant(factory()->NewStringFromAsciiChecked(
                         "String Iterator.prototype.next", TENURED)),
                     iterator);
     Return(result);  // Never reached.
   }
 }
 
 }  // namespace internal
 }  // namespace v8