Fix i939: Patmat shouldn't compare selectors to module TypeTrees.
Fix i939: Patmat shouldn't compare selectors to module TypeTrees.

It would be great to augment Ycheck to ensure that some trees are not
permitted, e.g. TypeTree cannot be a stat in a block or a receiver of
 a call.

Currently this is `ensured` by backend by failing.

lampepfl/dotty,som-snytt/dotty,som-snytt/dotty,dotty-staging/dotty,som-snytt/dotty,dotty-staging/dotty,som-snytt/dotty,densh/dotty,lampepfl/dotty,sjrd/dotty,dotty-staging/dotty,lampepfl/dotty,som-snytt/dotty,sjrd/dotty,sjrd/dotty,lampepfl/dotty,sjrd/dotty,densh/dotty,sjrd/dotty,lampepfl/dotty,dotty-staging/dotty,dotty-staging/dotty