Don't allow phi equivalences along back-edges.

lib/compiler/implementation/ssa/codegen_helpers.dart

 // 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.
 
 /**
  * Instead of emitting each SSA instruction with a temporary variable
  * mark instructions that can be emitted at their use-site.
  * For example, in:
  *   t0 = 4;
  *   t1 = 3;
@@ skipping 422 matching lines @@
   // All binary operators (excluding assignment) are left associative.
   int get precedence() => left;
 }
 
 class PhiEquivalator {
   final Equivalence<HPhi> equivalence;
   final Map<HPhi, String> logicalOperations;
   PhiEquivalator(this.equivalence, this.logicalOperations);
 
   void analyzeGraph(HGraph graph) {
-    graph.blocks.forEach((HBasicBlock block) => analyzeBlock(block));
+    graph.blocks.forEach(analyzeBlock);
   }
 
   void analyzeBlock(HBasicBlock block) {
     for (HPhi phi = block.phis.first; phi !== null; phi = phi.next) {
       if (!logicalOperations.containsKey(phi) &&
           phi.usedBy.length == 1 &&
-          phi.usedBy[0] is HPhi) {
+          phi.usedBy[0] is HPhi &&
+          phi.block.id < phi.usedBy[0].block.id) {
         equivalence.makeEquivalent(phi, phi.usedBy[0]);
       }
     }
   }
 }
 
 
 /**
  * Try to figure out which phis can be represented by the same temporary
  * variable, to avoid creating a new variable for each phi.
@@ skipping 31 matching lines @@
     // improving the performance of future lookups.
     T root = getRepresentative(parent);
     if (root !== parent) representative[element] = root;
     return root;
   }
 
   bool areEquivalent(T a, T b) {
     return getRepresentative(a) === getRepresentative(b);
   }
 }