Use knowledge of known subclasses of local classes more to enable more tailrec transforms

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Follow up with ideas raised during review of:

scala/scala#4652 (comment)

WIP:

retronym/scala@ticket/9408...ticket/9408-extra

Original Ticket:
#9408

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Imported From: https://issues.scala-lang.org/browse/SI-9414?orig=1
Reporter: @retronym
See #9408