Don't allow phi equivalences along back-edges.

frog/tests/leg/ssa_phi_codegen_test.dart

 // Copyright (c) 2011, 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 parameters keep their names in the output.
 
 #import("compiler_helper.dart");
 
 final String TEST_ONE = @"""
 void foo(bar) {
   var a = 1;
@@ skipping 48 matching lines @@
   RegExp regexp = const RegExp(@"a = 2");
   Expect.isTrue(regexp.hasMatch(generated));
 
   regexp = const RegExp(@"a = 3");
   Expect.isTrue(regexp.hasMatch(generated));
 
   regexp = const RegExp(@"print\(a\)");
   Expect.isTrue(regexp.hasMatch(generated));
 
   generated = compile(TEST_TWO, 'main');
+  // TODO(ngeoffray): Add live range analysis to the codegen
+  // to make this test pass.
   regexp = new RegExp("t = \\(?$anyIdentifier +");
-  Expect.isTrue(regexp.hasMatch(generated));
+  Expect.isFalse(regexp.hasMatch(generated));
 
   // TODO(ngeoffray): Add live range analysis to the codegen
   // to make this test pass.
   regexp = new RegExp("i = \\(?$anyIdentifier +");
   Expect.isFalse(regexp.hasMatch(generated));
 
   generated = compile(TEST_THREE, 'foo');
 
   // Check that we don't have 'val = val'.
   regexp = const RegExp("val = val;");
   Expect.isTrue(!regexp.hasMatch(generated));
 
   regexp = const RegExp("return val");
   Expect.isTrue(regexp.hasMatch(generated));
   // Check that a store just after the declaration of the local
   // only generates one instruction.
   regexp = const RegExp(@"var val = 42");
   Expect.isTrue(regexp.hasMatch(generated));
 
   generated = compile(TEST_FOUR, 'foo');
 
   regexp = const RegExp("cond1 = cond1;");
   Expect.isTrue(!regexp.hasMatch(generated));
 
   regexp = const RegExp("cond2 = cond2;");
   Expect.isTrue(!regexp.hasMatch(generated));
 }