[crankshaft] Fix IsClassOfTest helper method

test/mjsunit/regress/regress-5749.js

+// 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.
+
+// Flags: --allow-natives-syntax
+
+function f(x) {
+    (x ** 1) === '';
+}
+f();
+f();
+f();
+%OptimizeFunctionOnNextCall(f);
+f();
+
+function g(x) {
+    '' === (x ** 1);
+}
+g();
+g();
+g();
+%OptimizeFunctionOnNextCall(g);
+g();