Fix loss-of-SpecConstr bug
This bug, reported in Trac #13623 has been present since commit b8b3e30a Author: Edward Z. Yang <email@example.com> 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.
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