Fix NotNullInfo in multi-case match

[noti0na1]

When we compute afterPatternContext, the wrong ctx is passed to the function.

def f(s: AnyRef | Null) = s match
  case null => 0
  case s: String => s.length
  case _ =>
    val a: AnyRef = s
    a.toString.length

Will be useful for #23566

[olhotak]

LGTM