Revert "VM: Simplify lowering of is-tests."

runtime/vm/simulator_dbc.cc

Index: runtime/vm/simulator_dbc.cc
diff --git a/runtime/vm/simulator_dbc.cc b/runtime/vm/simulator_dbc.cc
index e0457de4433834ecb844eaaef15c95b4fc7f310a..d16a5a375869ea3c06faeb2d7826d0b627feec63 100644
--- a/runtime/vm/simulator_dbc.cc
+++ b/runtime/vm/simulator_dbc.cc
@@ -2924,7 +2924,7 @@ RawObject* Simulator::Call(const Code& code,
   }
 
   {
-    BYTECODE(InstanceOf, 0);  // Stack: instance, type args, type, cache
+    BYTECODE(InstanceOf, A);  // Stack: instance, type args, type, cache
     RawInstance* instance = static_cast<RawInstance*>(SP[-3]);
     RawTypeArguments* instantiator_type_arguments =
         static_cast<RawTypeArguments*>(SP[-2]);
@@ -2984,6 +2984,9 @@ RawObject* Simulator::Call(const Code& code,
 
   InstanceOfOk:
     SP -= 3;
+    if (rA) {  // Negate result.
+      SP[0] = (SP[0] == true_value) ? false_value : true_value;
+    }
     DISPATCH();
   }