Reapply "Fix misoptimization of 'is' test""

runtime/vm/intermediate_language_x64.cc

Index: runtime/vm/intermediate_language_x64.cc
diff --git a/runtime/vm/intermediate_language_x64.cc b/runtime/vm/intermediate_language_x64.cc
index 8f22f334899c7eff30a60b316c5d44a5b68c12b0..112cdc6b971192080ad12cbcbcfd2bfa357717e3 100644
--- a/runtime/vm/intermediate_language_x64.cc
+++ b/runtime/vm/intermediate_language_x64.cc
@@ -684,8 +684,11 @@ Condition TestCidsInstr::EmitComparisonCode(FlowGraphCompiler* compiler,
     __ cmpq(cid_reg, Immediate(test_cid));
     __ j(EQUAL, result ? labels.true_label : labels.false_label);
   }
-  // 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) {
       __ jmp(target);