Allow merging of monomorphic accesses to tracked fields.

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 365 matching lines @@
       INC_STAT(removed_);
     } else if (value != instr) {
       instr->DeleteAndReplaceWith(value);
       INC_STAT(redundant_);
     }
   }
 
   void ReduceLoadNamedField(HLoadNamedField* instr) {
     // Reduce a load of the map field when it is known to be a constant.
     if (!IsMapAccess(instr->access())) {
-      // Check if we introduce a map here.
-      if (!instr->map().IsNull()) {
-        Insert(instr, instr, instr->map());
+      // Check if we introduce field maps here.
+      if (instr->map_set().size() != 0) {
+        Insert(instr, instr, instr->map_set().Copy(phase_->zone()));
       }
       return;
     }
 
     HValue* object = instr->object()->ActualValue();
     MapSet maps = FindMaps(object);
     if (maps == NULL || maps->size() != 1) return;  // Not a constant.
 
     Unique<Map> map = maps->at(0);
     HConstant* constant = HConstant::CreateAndInsertBefore(
@@ skipping 343 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