Reapply "Fix misoptimization of 'is' test""

runtime/vm/intermediate_language_mips.cc

Index: runtime/vm/intermediate_language_mips.cc
diff --git a/runtime/vm/intermediate_language_mips.cc b/runtime/vm/intermediate_language_mips.cc
index a4fcff5b107c94090e981220736408ea7fe2a3b1..c1daa06c351d481c14ef9c3810953aed2fc37292 100644
--- a/runtime/vm/intermediate_language_mips.cc
+++ b/runtime/vm/intermediate_language_mips.cc
@@ -894,8 +894,11 @@ Condition TestCidsInstr::EmitComparisonCode(FlowGraphCompiler* compiler,
     __ BranchEqual(cid_reg, Immediate(test_cid),
                    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) {
       __ b(target);