Commit 714bebff removed
a check in the bytecode compiler that caught illegal uses of unboxed
tuples (and now sums) in case alternatives, which causes the program
in #14608 to panic. This restores the check (using modern,
levity-polymorphic vocabulary).

Test Plan: make test TEST=T14608

Reviewers: hvr, bgamari, dfeuer, simonpj

Reviewed By: dfeuer, simonpj

Subscribers: simonpj, rwbarton, thomie, carter

GHC Trac Issues: #14608

Differential Revision: https://phabricator.haskell.org/D4276

(cherry picked from commit ecff651f)