[turbofan] Assign proper type to %ClassOf/%_ClassOf.

src/compiler/typer.cc

Index: src/compiler/typer.cc
diff --git a/src/compiler/typer.cc b/src/compiler/typer.cc
index 0802c3aebacf587f14bf33758316f5347b4bf933..b2994a2721a42aa127381eee9ae7a72fb4ab1030 100644
--- a/src/compiler/typer.cc
+++ b/src/compiler/typer.cc
@@ -1561,6 +1561,9 @@ Type* Typer::Visitor::TypeJSCallRuntime(Node* node) {
       return TypeUnaryOp(node, ToObject);
     case Runtime::kInlineToString:
       return TypeUnaryOp(node, ToString);
+    case Runtime::kClassOf:
+    case Runtime::kInlineClassOf:
+      return Type::Union(Type::InternalizedString(), Type::Null(), zone());
     case Runtime::kHasInPrototypeChain:
       return Type::Boolean();
     default: