Replace guarded list length field loads with constants

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
diff --git a/runtime/vm/intermediate_language.cc b/runtime/vm/intermediate_language.cc
index 5ea37aa7cca3e7cb81557f082958a62fef01970d..6f183d89f1c6e45845bed6fee8735f9943366035 100644
--- a/runtime/vm/intermediate_language.cc
+++ b/runtime/vm/intermediate_language.cc
@@ -1368,6 +1368,16 @@ Definition* LoadFieldInstr::Canonicalize(FlowGraph* flow_graph) {
       IsFixedLengthArrayCid(call->Type()->ToCid())) {
     return call->ArgumentAt(1);
   }
+  // For arrays with guarded lengths, replace the length load
+  // with a constant.
+  LoadFieldInstr* load_array = instance()->definition()->AsLoadField();
+  if (load_array != NULL) {
+    const Field* field = load_array->field();
+    if ((field != NULL) && (field->guarded_list_length() >= 0)) {
+      return flow_graph->GetConstant(
+          Smi::Handle(Smi::New(field->guarded_list_length())));
+    }
+  }
   return this;
 }