Fix Trac #3831: blowup in SpecConstr
It turned out that there were two bugs. First, we were getting an exponential number of specialisations when we had a deep nest of join points. See Note [Avoiding exponential blowup]. I fixed this by dividing sc_count (in ScEnv) by the number of specialisations when recursing. Crude but effective. Second, when making specialisations I was looking at the result of applying specExpr to the RHS of the function, whereas I should have been looking at the original RHS. See Note [Specialise original body]. There's a tantalising missed opportunity here, though. In this example (recorded as a test simplCore/should_compile/T3831), each join point has *exactly one* call pattern, so we should really just specialise for that alone, in which case there's zero code-blow-up. In particular, we don't need the *original* RHS at all. I need to think more about how to exploit this. But the blowup is now limited, so compiling terminfo with -O2 works again.