Fix constructor-specialisation bug

Fix constructor-specialisation bug

The constructor-specialisation optimisation was not dealing with the case
of
	(letrec ... in f) a1 a2

We need to apply constructor specialisation in the letrec; previously
we were leaving it untouched on the grounds that the function part of
an application is almost always a variable.

But in fact, float-in immediately precedes SpecConstr, so we can get
these odd-looking applications.