[builtins] migrate C++ String Iterator builtins to baseline TurboFan

src/objects-debug.cc

 // Copyright 2012 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/objects.h"
 
 #include "src/bootstrapper.h"
 #include "src/disasm.h"
 #include "src/disassembler.h"
 #include "src/field-type.h"
@@ skipping 768 matching lines @@
   CHECK(IsJSWeakMap());
   JSObjectVerify();
   VerifyHeapPointer(table());
   CHECK(table()->IsHashTable() || table()->IsUndefined(GetIsolate()));
 }
 
 void JSStringIterator::JSStringIteratorVerify() {
   CHECK(IsJSStringIterator());
   JSObjectVerify();
   CHECK(string()->IsString());
+
+  InstanceType instance_type = map()->instance_type();
+
+  static_assert(kStringRepresentationMask == 3, "2-bit string rep mask");
+  static_assert((kSeqStringTag & 1) == 0, "SeqString rep low bit is 0");
+  static_assert((kExternalStringTag & 1) == 0, "SlicedString rep low bit is 0");
+  CHECK_EQ(instance_type & 1, 0);
+
   CHECK_GE(index(), 0);
   CHECK_LE(index(), String::kMaxLength);
 }
 
 void JSWeakSet::JSWeakSetVerify() {
   CHECK(IsJSWeakSet());
   JSObjectVerify();
   VerifyHeapPointer(table());
   CHECK(table()->IsHashTable() || table()->IsUndefined(GetIsolate()));
 }
@@ skipping 562 matching lines @@
 
   // Both are done at the same time.
   CHECK_EQ(new_it.done(), old_it.done());
 }
 
 
 #endif  // DEBUG
 
 }  // namespace internal
 }  // namespace v8