Fix fast intrinsics for getters and setters; previous strategy was too conservative.

runtime/vm/flow_graph_compiler.cc

Index: runtime/vm/flow_graph_compiler.cc
diff --git a/runtime/vm/flow_graph_compiler.cc b/runtime/vm/flow_graph_compiler.cc
index a0f7d802c37cb7ff7e7672e6b30a3b47018474e6..d904c864257667782dd6de276fc95f86797e1d62 100644
--- a/runtime/vm/flow_graph_compiler.cc
+++ b/runtime/vm/flow_graph_compiler.cc
@@ -1109,7 +1109,7 @@ bool FlowGraphCompiler::TryIntrinsify() {
           *return_node.value()->AsLoadInstanceFieldNode();
       // Only intrinsify getter if the field cannot contain a mutable double.
       // Reading from a mutable double box requires allocating a fresh double.
-      if (load_node.field().guarded_cid() == kDynamicCid) {
+      if (!IsPotentialUnboxedField(load_node.field())) {
         GenerateInlinedGetter(load_node.field().Offset());
         return !FLAG_use_field_guards;
       }