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

src/objects-debug.cc

Index: src/objects-debug.cc
diff --git a/src/objects-debug.cc b/src/objects-debug.cc
index 197d979258ffcf9fd386f38b9db4d545a1d2f09f..423d64c662a1af593e321a741a88d2e7e1e939d0 100644
--- a/src/objects-debug.cc
+++ b/src/objects-debug.cc
@@ -786,6 +786,14 @@ 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);
 }