[turbofan] Properly check call arity for Object.is(o,o).

Bug: chromium:771971, v8:6882
Change-Id: Id1a602306bc89a5f96e180f70d6f713015d2dbb6
Reviewed-on: https://chromium-review.googlesource.com/702834Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48329}

src/compiler/js-builtin-reducer.cc

@@ -2502,7 +2502,7 @@ Reduction JSBuiltinReducer::ReduceObjectIs(Node* node) {
   // SameValue simplified operator (and also a StrictEqual simplified
   // operator) and create unified handling in SimplifiedLowering.
   JSCallReduction r(node);
-  if (r.left() == r.right()) {
+  if (r.GetJSCallArity() == 2 && r.left() == r.right()) {
     // Object.is(x,x) => #true
     Node* value = jsgraph()->TrueConstant();
     return Replace(value);

test/mjsunit/regress/regress-crbug-771971.js

+// 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 f() { Object.is(); }
+
+f();
+f();
+%OptimizeFunctionOnNextCall(f);
+f();