Constant propagator should revisit phis when it visits predecessor block.

tests/language/constant_propagation_phis_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.
+
+// Test that constant propagation correctly updates phis when predecessor's
+// reachability changes.
+
+final keys = const ["keyA"];
+final values = const ["a"];
+
+main() {
+  for (var i = 0; i < 10000; i++) test(keys[0]);
+}
+
+test(key) {
+  var ref = key2value(key);
+  Expect.equals("a", (ref == null) ? "-" : ref);
+}
+
+key2value(key) {
+  var index = indexOf(keys, key);
+  return (index == -1) ? null : values[index];
+}
+
+indexOf(keys, key) {
+  for (var i = keys.length - 1; i >= 0; i--) {
+    var equals = keys[i] == key;
+    if (equals) return i;
+  }
+  return -1;
+}