[turbofan] Introduce dedicated StringIndexOf operator.

test/mjsunit/regress/regress-crbug-685680.js

Index: test/mjsunit/regress/regress-crbug-685680.js
diff --git a/test/mjsunit/regress/regress-crbug-685680.js b/test/mjsunit/regress/regress-crbug-685680.js
new file mode 100644
index 0000000000000000000000000000000000000000..3c23e414cbc11cc3650e3b73ec0dc97e0b282d7f
--- /dev/null
+++ b/test/mjsunit/regress/regress-crbug-685680.js
@@ -0,0 +1,15 @@
+// Copyright 2017 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 foo(s) {
+  s = s + '0123456789012';
+  return s.indexOf('0');
+}
+
+assertEquals(0, foo('0'));
+assertEquals(0, foo('0'));
+%OptimizeFunctionOnNextCall(foo);
+assertEquals(0, foo('0'));