Fix register allocation bug in building use intervals.

tests/language/regress_11800_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 correct register allocation with a value used twice at the same
+// instruction.
+test(List a, int v) {
+  a[v] = v;
+}
+
+main() {
+  var list = new List(2);
+  for (var i = 0; i < 20; i++) test(list, 1);
+  Expect.equals(null, list[0]);
+  Expect.equals(1, list[1]);
+}