TcSimplify.decideQuantification is truly a tricky function!
Trac #13509 showed that we were being over-eager with defaulting
of runtime-rep variables (levity polymorphism), which meant that
a program was wrongly rejected, and with a very odd error message
(c.f. Trac #13530)

I spent an unreasonably long time figuring out how to fix this
in a decent way, and ended up with a major refactoring of
decideQuantification, with a kock-on effect in simplifyInfer.

It is at least a bit more comprehensible now; but I still
can't say I like it.