We run phases in-between GVN and code motion, so we have to handle cases in the SsaCodeMotion phase…

sdk/lib/_internal/compiler/implementation/ssa/optimize.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.
 
 part of ssa;
 
 abstract class OptimizationPhase {
   String get name;
   void visitGraph(HGraph graph);
 }
@@ skipping 1430 matching lines @@
 
       bool canBeMoved = true;
       for (final HInstruction input in current.inputs) {
         if (input.block == block) {
           canBeMoved = false;
           break;
         }
       }
       if (!canBeMoved) continue;
 
-      // This is safe because we are running after GVN.
-      // TODO(ngeoffray): ensure GVN has been run.
-      set_.add(current);
+      HInstruction existing = set_.lookup(current);
+      if (existing == null) {
+        set_.add(current);
+      } else {
+        block.rewriteWithBetterUser(current, existing);
+        block.remove(current);
+      }
     }
   }
 }
 
 class SsaTypeConversionInserter extends HBaseVisitor
     implements OptimizationPhase {
   final String name = "SsaTypeconversionInserter";
   final Compiler compiler;
 
   SsaTypeConversionInserter(this.compiler);
@@ skipping 473 matching lines @@
 
     keyedValues.forEach((receiver, values) {
       result.keyedValues[receiver] =
           new Map<HInstruction, HInstruction>.from(values);
     });
 
     result.nonEscapingReceivers.addAll(nonEscapingReceivers);
     return result;
   }
 }