Reland "Add a simple dominator based redundancy elimination" and fix a register allocation bug.

runtime/vm/flow_graph_allocator.cc

 // 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.
 
 #include "vm/flow_graph_allocator.h"
 
 #include "vm/bit_vector.h"
 #include "vm/intermediate_language.h"
 #include "vm/il_printer.h"
 #include "vm/flow_graph.h"
@@ skipping 1568 matching lines @@
       // This is an active interval.
       if (allocated->vreg() < 0) {
         // This register blocked by an interval that
         // can't be spilled.
         return false;
       }
 
       const UsePosition* use =
           allocated->finger()->FirstRegisterBeneficialUse(unallocated->Start());
 
-      if ((use != NULL) && ((use->pos() - start) <= 1)) {
+      if ((use != NULL) && ((ToInstructionStart(use->pos()) - start) <= 1)) {
         // This register is blocked by interval that is used
         // as register in the current instruction and can't
         // be spilled.
         return false;
       }
 
       const intptr_t use_pos = (use != NULL) ? use->pos()
                                              : allocated->End();
 
       if (use_pos < free_until) free_until = use_pos;
@@ skipping 433 matching lines @@
     OS::Print("-- [after ssa allocator] ir [%s] -------------\n",
               function.ToFullyQualifiedCString());
     FlowGraphPrinter printer(flow_graph_, true);
     printer.PrintBlocks();
     OS::Print("----------------------------------------------\n");
   }
 }
 
 
 }  // namespace dart