Bugfix in check elimination with a regression test.

src/hydrogen-check-elimination.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 320 matching lines @@
           // Narrow set of maps in the second check maps instruction.
           HGraph* graph = instr->block()->graph();
           if (entry->check_ != NULL &&
               entry->check_->block() == instr->block() &&
               entry->check_->IsCheckMaps()) {
             // There is a check in the same block so replace it with a more
             // strict check and eliminate the second check entirely.
             HCheckMaps* check = HCheckMaps::cast(entry->check_);
             TRACE(("CheckMaps #%d at B%d narrowed\n", check->id(),
                 check->block()->block_id()));
+            // Update map set and ensure that the check is alive.
             check->set_map_set(intersection, graph->zone());
+            check->ClearFlag(HValue::kIsDead);
             TRACE(("Replacing redundant CheckMaps #%d at B%d with #%d\n",
                 instr->id(), instr->block()->block_id(), entry->check_->id()));
             instr->DeleteAndReplaceWith(entry->check_);
           } else {
             TRACE(("CheckMaps #%d at B%d narrowed\n", instr->id(),
                 instr->block()->block_id()));
             instr->set_map_set(intersection, graph->zone());
             entry->check_ = instr;
           }
 
@@ skipping 374 matching lines @@
   PRINT_STAT(removed_cho);
   PRINT_STAT(narrowed);
   PRINT_STAT(loads);
   PRINT_STAT(empty);
   PRINT_STAT(compares_true);
   PRINT_STAT(compares_false);
   PRINT_STAT(transitions);
 }
 
 } }  // namespace v8::internal