We don't need to update StgCase's AltType, because it's already set
correctly in `CoreToStg.mkStgAltType`, so we can just remove extra
argument passing and return values.

(I think this is a useful refactoring because it makes it clear that we
don't need to update AltTypes)

Reviewers: austin, bgamari, simonpj

Reviewed By: bgamari, simonpj

Subscribers: simonpj, thomie

Differential Revision: https://phabricator.haskell.org/D1403