VM: Fix identical comparisons with bigints.

tests/language/vm/regress_14903_test.dart

+// Copyright (c) 2013, 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.
+// VMOptions=--optimization-counter-threshold=10 
+
+import "package:expect/expect.dart";
+
+// Test identical comparisons in optimized code. Registers must be preserved
+// when calling the runtime.
+
+cmp(a, b, c) {
+  var v = c + 1;
+  var w = v + 1;
+  var x = w + 1;
+  var y = x + 1;
+  var z = y + 1;
+  if (identical(a, b)) {
+    c++;
+  }
+  return c + v + w + x + y + z;
+}
+
+
+main() {
+  var big = 100000000000000000000000000000000000;
+  var before = cmp(big, big, 0);
+  Expect.equals(16, before);
+  for (var i = 0; i < 20; i++) cmp(big, big + 1, 0);
+  Expect.equals(before, cmp(big, big, 0));
+}
+