use range to avoid duplicate code

Author: liufengyun

compiler/src/dotty/tools/dotc/transform/IsInstanceOfChecker.scala

@@ -93,9 +93,7 @@ object Checkable {
         case tp: TypeRef if tp.underlying.isInstanceOf[TypeBounds] =>
           val lo = this(tp.info.loBound)
           val hi = this(tp.info.hiBound)
-          if (variance == 0) range(lo, hi)
-          else if (variance == 1) hi
-          else lo
+          range(lo, hi)
         case _ =>
           mapOver(tp)
       }