Remove LoadFieldInstr with no uses

runtime/vm/intermediate_language.cc

 // Copyright (c) 2013, 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/intermediate_language.h"
 
 #include "vm/bigint_operations.h"
 #include "vm/bit_vector.h"
 #include "vm/dart_entry.h"
 #include "vm/flow_graph_allocator.h"
@@ skipping 1336 matching lines @@
   }
 }
 
 
 Definition* ConstantInstr::Canonicalize(FlowGraph* flow_graph) {
   return HasUses() ? this : NULL;
 }
 
 
 Definition* LoadFieldInstr::Canonicalize(FlowGraph* flow_graph) {
+  if (!HasUses()) return NULL;
   if (!IsImmutableLengthLoad()) return this;
 
   // For fixed length arrays if the array is the result of a known constructor
   // call we can replace the length load with the length argument passed to
   // the constructor.
   StaticCallInstr* call = instance()->definition()->AsStaticCall();
   if ((call != NULL) &&
       call->is_known_list_constructor() &&
       IsFixedLengthArrayCid(call->Type()->ToCid())) {
     return call->ArgumentAt(1);
@@ skipping 1283 matching lines @@
     default:
       UNREACHABLE();
   }
   return kPowRuntimeEntry;
 }
 
 
 #undef __
 
 }  // namespace dart