Fix bad code-generation for nested-ifs in for loops.

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 287 matching lines @@
     }
 
     // If the [thenBlock] is already a control flow operation, and does not
     // have any statement and its join block is [end], we can emit a
     // sequence of control flow operation.
     if (controlFlowOperators.contains(thenBlock.last)) {
       HIf otherIf = thenBlock.last;
       if (otherIf.joinBlock !== end) {
         // This could be a join block that just feeds into our join block.
         HBasicBlock otherJoin = otherIf.joinBlock;
+        if (otherJoin.first != otherJoin.last) return;
         if (otherJoin.successors.length != 1) return;
         if (otherJoin.successors[0] != end) return;
         if (otherJoin.phis.isEmpty()) return;
         if (otherJoin.phis.first !== otherJoin.phis.last) return;
         HPhi otherPhi = otherJoin.phis.first;
         if (thenInput != otherPhi) return;
         if (elseInput != otherPhi.inputs[1]) return;
       }
       if (hasAnyStatement(thenBlock, otherIf)) return;
     } else {
@@ skipping 21 matching lines @@
     }
 
     // If [thenInput] is defined in the first predecessor, then it is only used
     // by [phi] and can be generated at use site.
     if (thenInput.block === end.predecessors[0]) {
       assert(thenInput.usedBy.length == 1);
       markAsGenerateAtUseSite(thenInput);
     }
   }
 }