Don't allow phi equivalences along back-edges.

frog/tests/leg_only/regression_2913_test.dart

Index: frog/tests/leg_only/regression_2913_test.dart
diff --git a/frog/tests/leg_only/regression_2913_test.dart b/frog/tests/leg_only/regression_2913_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..0099a6a436ea4465c7fa3fe9b60d6b8b5dbf9d83
--- /dev/null
+++ b/frog/tests/leg_only/regression_2913_test.dart
@@ -0,0 +1,23 @@
+// Copyright (c) 2012, 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.
+
+main() {
+  var a = 10;
+  var b = 11;
+
+  for (var i = 0; i < 1; i++) {
+    print('--------');
+    print('a $a');
+    print('b $b');
+    var t = 1;
+    if (i < 20) {
+      t = 2;
+    }
+
+    b = a;
+    a = t;
+  }
+  Expect.equals(2, a);
+  Expect.equals(10, b);
+}