[turbofan] Introduce a dedicated StringCharAt operator.

src/compiler/typer.cc

 // 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.
 
 #include "src/compiler/typer.h"
 
 #include <iomanip>
 
 #include "src/base/flags.h"
 #include "src/bootstrapper.h"
@@ skipping 1663 matching lines @@
 }
 
 Type* Typer::Visitor::StringFromCharCodeTyper(Type* type, Typer* t) {
   return Type::String();
 }
 
 Type* Typer::Visitor::StringFromCodePointTyper(Type* type, Typer* t) {
   return Type::String();
 }
 
+Type* Typer::Visitor::TypeStringCharAt(Node* node) { return Type::String(); }
+
 Type* Typer::Visitor::TypeStringCharCodeAt(Node* node) {
   return typer_->cache_.kUint16;
 }
 
 Type* Typer::Visitor::TypeStringFromCharCode(Node* node) {
   return TypeUnaryOp(node, StringFromCharCodeTyper);
 }
 
 Type* Typer::Visitor::TypeStringFromCodePoint(Node* node) {
   return TypeUnaryOp(node, StringFromCodePointTyper);
@@ skipping 161 matching lines @@
 Type* Typer::Visitor::TypeConstant(Handle<Object> value) {
   if (Type::IsInteger(*value)) {
     return Type::Range(value->Number(), value->Number(), zone());
   }
   return Type::NewConstant(value, zone());
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8