[turbofan] Introduce a dedicated StringCharAt operator.

src/code-stub-assembler.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/code-stub-assembler.h"
 #include "src/code-factory.h"
 #include "src/frames-inl.h"
 #include "src/frames.h"
 
 namespace v8 {
 namespace internal {
@@ skipping 2993 matching lines @@
 
 Node* CodeStubAssembler::IsUnseededNumberDictionary(Node* object) {
   return WordEqual(LoadMap(object),
                    LoadRoot(Heap::kUnseededNumberDictionaryMapRootIndex));
 }
 
 Node* CodeStubAssembler::IsJSFunction(Node* object) {
   return HasInstanceType(object, JS_FUNCTION_TYPE);
 }
 
-Node* CodeStubAssembler::StringCharCodeAt(Node* string, Node* index) {
+Node* CodeStubAssembler::StringCharCodeAt(Node* string, Node* index,
+                                          ParameterMode parameter_mode) {
   CSA_ASSERT(this, IsString(string));
   // Translate the {index} into a Word.
-  index = SmiToWord(index);
+  index = ParameterToWord(index, parameter_mode);
 
   // We may need to loop in case of cons or sliced strings.
   Variable var_index(this, MachineType::PointerRepresentation());
   Variable var_result(this, MachineRepresentation::kWord32);
   Variable var_string(this, MachineRepresentation::kTagged);
   Variable* loop_vars[] = {&var_index, &var_string};
   Label done_loop(this, &var_result), loop(this, 2, loop_vars);
   var_string.Bind(string);
   var_index.Bind(index);
   Goto(&loop);
@@ skipping 5339 matching lines @@
                        Heap::kUndefinedValueRootIndex);
   StoreObjectFieldRoot(result, PromiseReactionJobInfo::kDebugNameOffset,
                        Heap::kUndefinedValueRootIndex);
   StoreObjectFieldNoWriteBarrier(result, PromiseReactionJobInfo::kContextOffset,
                                  context);
   return result;
 }
 
 }  // namespace internal
 }  // namespace v8