[runtime] Add fast-paths for common conversion methods

src/regexp/regexp-utils.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/regexp/regexp-utils.h"
 
 #include "src/factory.h"
 #include "src/isolate.h"
 #include "src/objects-inl.h"
 #include "src/regexp/jsregexp.h"
@@ skipping 156 matching lines @@
     Isolate* isolate, Handle<JSReceiver> regexp, Handle<String> string,
     bool unicode) {
   Handle<Object> last_index_obj;
   ASSIGN_RETURN_ON_EXCEPTION(
       isolate, last_index_obj,
       Object::GetProperty(regexp, isolate->factory()->lastIndex_string()),
       Object);
 
   ASSIGN_RETURN_ON_EXCEPTION(isolate, last_index_obj,
                              Object::ToLength(isolate, last_index_obj), Object);
-
-  const int last_index = Handle<Smi>::cast(last_index_obj)->value();
+  const int last_index = PositiveNumberToUint32(*last_index_obj);
   const int new_last_index =
       AdvanceStringIndex(isolate, string, last_index, unicode);
 
   return SetLastIndex(isolate, regexp, new_last_index);
 }
 
 }  // namespace internal
 }  // namespace v8