Fix primitive rewriting

Author: felixmulder

src/dotty/tools/dotc/transform/IsInstanceOfEvaluator.scala

@@ -82,7 +82,7 @@ class IsInstanceOfEvaluator extends MiniPhaseTransform { thisTransformer =>
      *  `srutinee.isInstanceOf[Selector]` if `scrutinee eq null`
      */
     def rewrite(tree: Select, to: Boolean): Tree =
-      if (!to || tree.qualifier.tpe.widen.isPrimitiveValueType)
+      if (!to || !tree.qualifier.tpe.widen.derivesFrom(defn.AnyRefAlias))
         Literal(Constant(to))
       else
         Apply(tree.qualifier.select(defn.Object_ne), List(Literal(Constant(null))))