Note [Always analyse in virgin pass]

Tricky point: make sure that we analyse in the 'virgin' pass. Consider
   rec { f acc x True  = f (...rec { g y = ...g... }...)
         f acc x False = acc }
In the virgin pass for 'f' we'll give 'f' a very strict (bottom) type.
That might mean that we analyse the sub-expression containing the
E = "...rec g..." stuff in a bottom demand.  Suppose we *didn't analyse*
E, but just returned botType.

Then in the *next* (non-virgin) iteration for 'f', we might analyse E
in a weaker demand, and that will trigger doing a fixpoint iteration
for g.  But *because it's not the virgin pass* we won't start g's
iteration at bottom.  Disaster.  (This happened in $sfibToList' of
nofib/spectral/fibheaps.)

So in the virgin pass we make sure that we do analyse the expression
at least once, to initialise its signatures.