Identify more statically resolved (effectively final) methods

[lrytz]

trafficstars

trait TravLike { def map = ... }
sealed trait List extends TravLike { ... } // map is not overridden
final case class ::
final case object Nil
(l: List).map // can be inlined

we need to know that

• the recevier is sealed
• what are the children of the receiver
• all children are final
• none of the children overrides map

https://github.com/scala/scala/blob/2.12.x/src/compiler/scala/tools/nsc/backend/jvm/opt/CallGraph.scala#L293

[som-snytt]

relatedly https://github.com/scala/scala/pull/10629 for tailrec