Reapply "Fix misoptimization of 'is' test""

runtime/vm/intermediate_language_arm64.cc

Index: runtime/vm/intermediate_language_arm64.cc
diff --git a/runtime/vm/intermediate_language_arm64.cc b/runtime/vm/intermediate_language_arm64.cc
index 5ed869920c2cb30f6b32df6522e9e86cfbacf22b..460699d88c9aea1ba77a665d300f49d12727b3db 100644
--- a/runtime/vm/intermediate_language_arm64.cc
+++ b/runtime/vm/intermediate_language_arm64.cc
@@ -682,8 +682,11 @@ Condition TestCidsInstr::EmitComparisonCode(FlowGraphCompiler* compiler,
     __ CompareImmediate(cid_reg, test_cid);
     __ b(result ? labels.true_label : labels.false_label, EQ);
   }
-  // No match found, deoptimize or false.
+  // No match found, deoptimize or default action.
   if (deopt == NULL) {
+    // If the cid is not in the list, jump to the opposite label from the cids
+    // that are in the list.  These must be all the same (see asserts in the
+    // constructor).
     Label* target = result ? labels.false_label : labels.true_label;
     if (target != labels.fall_through) {
       __ b(target);