Do not unify representational equalities

Authored by simonpj on May 18 2018, 7:58 AM.

Description

Do not unify representational equalities

This patch is an easy fix to Trac Trac #15144, which was caused
by accidentally unifying a representational equality in the
unflattener. (The main code in TcInteract was always careful
not to do so, but I'd missed the test in the unflattener.)

See Note [Do not unify representational equalities]
in TcInteract