[turbofan] Introduce a dedicated StringCharAt operator.

src/code-factory.h

Index: src/code-factory.h
diff --git a/src/code-factory.h b/src/code-factory.h
index e15317696e98b6ce9aa5e867a6dd7f8008795bde..33c7faad05905e6d480bd6fdb0df04c8539d2546 100644
--- a/src/code-factory.h
+++ b/src/code-factory.h
@@ -118,6 +118,7 @@ class V8_EXPORT_PRIVATE CodeFactory final {
 
   static Callable StringAdd(Isolate* isolate, StringAddFlags flags,
                             PretenureFlag pretenure_flag);
+  static Callable StringCharAt(Isolate* isolate);
   static Callable StringCompare(Isolate* isolate, Token::Value token);
   static Callable StringEqual(Isolate* isolate);
   static Callable StringNotEqual(Isolate* isolate);