This patch fixes Trac #12734 by prioritising the class-level
variants of equality constraints, namely (a~b) and (a~~b).

See comment:10 of Trac #12734 for a description of what
went wrong, and Note [Prioritise class equalities] in TcSMonad.

The fix is still not great, but it's a definite step forward, and
cures the particular problem.

Worth merging to 8.0.