This bug, reported in Trac #13623 has been present since

  commit b8b3e30a
  Author: Edward Z. Yang <ezyang@cs.stanford.edu>
  Date:   Fri Jun 24 11:03:47 2016 -0700

      Axe RecFlag on TyCons.

SpecConstr tries not to specialise indefinitely, and had a
limit (see Note [Limit recursive specialisation]) that made
use of info about whether or not a data constructor was
"recursive".  This info vanished in the above commit, making
the limit fire much more often -- and indeed it fired in this
test case, in a situation where specialisation is /highly/
desirable.

I refactored the test, to look instead at the number of
iterations of the loop of "and now specialise calls that
arise from the specialisation".  Actually less code, and
more robust.

I also added record field names to a couple of constructors,
and renamed RuleInfo to SpecInfo.