allow nullability flow typing even in presence of pattern match

[olhotak]

Nullability flow typing is conservatively disabled for mutable variables to which a write occurs nested inside a Tree other than some known ones, such as If and WhileDo. This is to prevent flow-sensitive reasoning for variables that are captured and written to in a closure. Pattern matches do not create a closure. This change enables nullability flow typing even for mutable variables that are written to inside a pattern match.

[noti0na1]

I'd like test this PR after #18212 is merged.

var s: String | Null = ???

s match 
  case null => s = ""
  case _ =>

val ss: String = s

Will this work?

[olhotak]

It should be possible to also do this for Try, at least for the try and catch, though I'm not sure about the finally.