Prioritise equalities when solving, incl deriveds

We already prioritise equalities when solving, but
Trac #14723 showed that we were not doing so consistently
enough, and as a result the type checker could go into a loop.
Yikes.

See Note [Prioritise equalities] in TcSMonad.

Fixng this bug changed the solve order enough to demonstrate
a problem with fundeps: Trac #14745.

(cherry picked from commit efba0546)