Summary:
In the fallthrough case when doing a subsumption case, we
need to deeply instantiate to remove any buried foralls in
the "actual" type.

Once this validates, please feel free to commit it; I may not
have the chance to do this on Tuesday. Back in full action on
Wed.

Test Plan: ./validate, typecheck/should_compiler/T11305

Reviewers: austin, bgamari, hvr

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1715

GHC Trac Issues: #11305