Fix optimistic BCE to back off after deopt

test/mjsunit/regress/regress-3176.js

+// Copyright 2014 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(a) {
+  var sum = 0;
+  for (var i = 0; i < 10; i++) {
+    sum += a[i];
+
+    if (i > 6) {
+      sum -= a[i - 4];
+      sum -= a[i - 5];
+    }
+  }
+  return sum;
+}
+
+var a = new Int32Array(10);
+
+foo(a);
+foo(a);
+%OptimizeFunctionOnNextCall(foo);
+foo(a);
+%OptimizeFunctionOnNextCall(foo);
+foo(a);
+assertOptimized(foo);