Fix #14228 by marking SumPats as non-irrefutable

`isIrrefutableHsPat` should always return `False` for unboxed sum
patterns (`SumPat`s), since they always have at least one other
corresponding pattern of the same arity (since the minimum arity for a
`SumPat` is 2). Failure to do so causes incorrect code to be generated
for pattern synonyms that use unboxed sums, as shown in #14228.

Test Plan: make test TEST=T14228

Reviewers: austin, bgamari, simonpj

Reviewed By: simonpj

Subscribers: simonpj, rwbarton, thomie

GHC Trac Issues: #14228

Differential Revision: https://phabricator.haskell.org/D3951