Fix misoptimization of 'is' test

tests/standalone/regress_29695_test.dart

+// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+//
+// Test that type tests are not misoptimized.
+// VMOptions=--optimization-counter-threshold=1000 --optimization-filter=IsAnInt
+
+main() {
+  train();
+  if (IsAnInt("This is not an int")) throw "oops";
+}
+
+// Prime the IC with things that are and are not ints.
+void train() {
+  int sum = 0;
+  for (int i = 0; i < 10000; i++) {
+    IsAnInt(42); // Smi - always goes first in the generated code.
+    IsAnInt(1 << 62); // Mint on 64 bit platforms.
+    IsAnInt(1 << 62);
+    IsAnInt(4200000000000000000000000000000000000); // BigInt
+    IsAnInt(4200000000000000000000000000000000000);
+    // This one that is not an int goes last in the IC because it is called
+    // less frequently.
+    IsAnInt(4.2);
+  }
+}
+
+int IsAnInt(Foo f) => f is int;