SpecConstr has -fspec-contr-count=N which limits the maximum
number of specialisations we make for any particular function.
But until now, if that limit was exceeded we discarded all the
candidates!  So adding a new specialisaiton opportunity (by
adding a new call site, or improving the optimiser) could result
in less specialisation and worse performance.

This patch instead picks the top N candidates, resulting in
less brittle behaviour.

See Note [Choosing patterns].