Cleanup: Make CheckClassId instruction more general so it

runtime/vm/flow_graph_compiler_x64.cc

Index: runtime/vm/flow_graph_compiler_x64.cc
diff --git a/runtime/vm/flow_graph_compiler_x64.cc b/runtime/vm/flow_graph_compiler_x64.cc
index b0d6f72e952b5391d24e98c88d822cde2f3a0348..35d199f45fef003999332b8b7b0c24dc4c7bf48c 100644
--- a/runtime/vm/flow_graph_compiler_x64.cc
+++ b/runtime/vm/flow_graph_compiler_x64.cc
@@ -1477,13 +1477,13 @@ int FlowGraphCompiler::EmitTestAndCallCheckCid(Label* next_label,
                                                int bias) {
   intptr_t cid_start = range.cid_start;
   intptr_t cid_end = range.cid_end;
-  if (cid_start == cid_end) {
+  if (range.IsSingleCid()) {
     __ cmpl(RDI, Immediate(cid_start - bias));
     __ j(NOT_EQUAL, next_label);
   } else {
     __ addl(RDI, Immediate(bias - cid_start));
     bias = cid_start;
-    __ cmpl(RDI, Immediate(cid_end - cid_start));
+    __ cmpl(RDI, Immediate(range.Extent()));
     __ j(ABOVE, next_label);  // Unsigned higher.
   }
   return bias;