Fix Trac #2486: restore the specialiser to a working state

Fix Trac #2486: restore the specialiser to a working state

In improving the specialiser's data reprsentaion, thus
  Mon Apr 28 16:57:11 GMT Daylight Time 2008  simonpj@microsoft.com
  * Fix Trac #1969: perfomance bug in the specialiser
I got the sense of a pair of filter functions back to front.
As a result, almost all specialisation opportunities were being
filtered out, and no specialisations were being generated.

Fortunately, dolio notice and reported Trac #2486.  The fix is
simple: put the filters the right way roud.