fix TypeBounds case in isNotNull

Author: olhotak

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -375,7 +375,7 @@ object Types extends TypeUtils {
       case tp: FlexibleType => false
       case tp: ClassInfo => !tp.cls.isNullableClass && !tp.isNothingType
       case tp: AppliedType => tp.superType.isNotNull
-      case tp: TypeBounds => tp.lo.isNotNull
+      case tp: TypeBounds => tp.hi.isNotNull
       case tp: TypeProxy => tp.underlying.isNotNull
       case AndType(tp1, tp2) => tp1.isNotNull || tp2.isNotNull
       case OrType(tp1, tp2) => tp1.isNotNull && tp2.isNotNull