Fix #15725 with an extra Sym

Summary:
We were adding a `Sym` to one argument in the `InstCo`
case of `optCoercion` but not another, leading to the two arguments
to misaligned when combined via `Trans`. This fixes the issue with
a well targeted use of `wrapSym`.

Test Plan: make test TEST=T15725

Reviewers: goldfire, ningning, bgamari

Reviewed By: goldfire, ningning

Subscribers: rwbarton, carter

GHC Trac Issues: #15725

Differential Revision: https://phabricator.haskell.org/D5217