Fix #2253 edge cases: recurse into refined type

[abgruszecki]

I managed to get some interesting results after #2257:

scala> trait T1; trait T2; sealed trait S; object O extends S with T2; object O2
defined trait T1
defined trait T2
defined trait S
defined module O
defined module O2
scala> def m(s: S { val x: T2 }) = s match { case _: T1 => ; }
-- [E028] Pattern Match Exhaustivity Warning: <console> ------------------------
7 |def m(s: S { val x: T2 }) = s match { case _: T1 => ; }
  |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |                            match may not be exhaustive.
  |
  |                            It would fail on: O

This should hopefully fix such cases. @liufengyun could you please review?

[liufengyun]

LGTM, thanks a lot @AleksanderBG 👍