[turbofan] Add support for String.prototype.charCodeAt/charAt.

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 "src/base/flags.h"
 #include "src/bootstrapper.h"
 #include "src/compilation-dependencies.h"
 #include "src/compiler/common-operator.h"
@@ skipping 1865 matching lines @@
   double min = type->Min();
   double max = type->Max();
   if (min == max) {
     uint32_t code = static_cast<uint32_t>(min) & String::kMaxUtf16CodeUnitU;
     Handle<String> string = f->LookupSingleCharacterStringFromCode(code);
     return Type::Constant(string, t->zone());
   }
   return Type::String();
 }
 
+Type* Typer::Visitor::TypeStringCharCodeAt(Node* node) {
+  // TODO(bmeurer): We could do better here based on inputs.
+  return Type::Range(0, kMaxUInt16, zone());
+}
+
 Type* Typer::Visitor::TypeStringFromCharCode(Node* node) {
   return TypeUnaryOp(node, StringFromCharCodeTyper);
 }
 
 Type* Typer::Visitor::TypeCheckBounds(Node* node) {
   // TODO(bmeurer): We could do better here based on the limit.
   return Type::Unsigned31();
 }
 
 Type* Typer::Visitor::TypeCheckNumber(Node* node) {
@@ skipping 804 matching lines @@
   }
   if (Type::IsInteger(*value)) {
     return Type::Range(value->Number(), value->Number(), zone());
   }
   return Type::Constant(value, zone());
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8