[turbofan] Introduce a dedicated StringCharAt operator.

src/code-stub-assembler.h

 // 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.
 
 #ifndef V8_CODE_STUB_ASSEMBLER_H_
 #define V8_CODE_STUB_ASSEMBLER_H_
 
 #include <functional>
 
 #include "src/compiler/code-assembler.h"
@@ skipping 651 matching lines @@
   Node* IsUnseededNumberDictionary(Node* object);
   Node* IsConstructorMap(Node* map);
   Node* IsJSFunction(Node* object);
 
   // ElementsKind helpers:
   Node* IsFastElementsKind(Node* elements_kind);
   Node* IsHoleyFastElementsKind(Node* elements_kind);
 
   // String helpers.
   // Load a character from a String (might flatten a ConsString).
-  Node* StringCharCodeAt(Node* string, Node* smi_index);
+  Node* StringCharCodeAt(Node* string, Node* index,
+                         ParameterMode parameter_mode = SMI_PARAMETERS);
   // Return the single character string with only {code}.
   Node* StringFromCharCode(Node* code);
   // Return a new string object which holds a substring containing the range
   // [from,to[ of string.  |from| and |to| are expected to be tagged.
   Node* SubString(Node* context, Node* string, Node* from, Node* to);
 
   // Return a new string object produced by concatenating |first| with |second|.
   Node* StringAdd(Node* context, Node* first, Node* second,
                   AllocationFlags flags = kNone);
 
@@ skipping 532 matching lines @@
   }
 #else
 #define CSA_SLOW_ASSERT(csa, x) ((void)0)
 #endif
 
 DEFINE_OPERATORS_FOR_FLAGS(CodeStubAssembler::AllocationFlags);
 
 }  // namespace internal
 }  // namespace v8
 #endif  // V8_CODE_STUB_ASSEMBLER_H_