Split GuardField into GuardFieldType and GuardFieldLength instructions.

runtime/vm/flow_graph_builder.cc

Index: runtime/vm/flow_graph_builder.cc
diff --git a/runtime/vm/flow_graph_builder.cc b/runtime/vm/flow_graph_builder.cc
index 9e9644f6fd2003b01980e2d63aa5066e4bd35ee3..f7804925d85302cf4ccaa29b7d0b35a350bcc24c 100644
--- a/runtime/vm/flow_graph_builder.cc
+++ b/runtime/vm/flow_graph_builder.cc
@@ -3174,11 +3174,19 @@ void EffectGraphVisitor::VisitStoreInstanceFieldNode(
   }
 
   store_value = Bind(BuildStoreExprTemp(store_value));
-  GuardFieldInstr* guard =
-      new GuardFieldInstr(store_value,
-                          node->field(),
-                          Isolate::Current()->GetNextDeoptId());
-  AddInstruction(guard);
+  GuardFieldClassInstr* guard_field_class =
+      new GuardFieldClassInstr(store_value,
+                               node->field(),
+                               Isolate::Current()->GetNextDeoptId());
+  AddInstruction(guard_field_class);
+
+  store_value = Bind(BuildLoadExprTemp());
+  GuardFieldLengthInstr* guard_field_length =
+      new GuardFieldLengthInstr(store_value,
+                                node->field(),
+                                Isolate::Current()->GetNextDeoptId());
+  AddInstruction(guard_field_length);
+
 
   store_value = Bind(BuildLoadExprTemp());
   StoreInstanceFieldInstr* store =