SynthesizePN: Produce less places in the synthesized net

If we solve ESSP and SSP only later, we may end up with less places in the
resulting net, because ESSP may need to calculate new regions while SSP only
choses from a region basis. However, the regions that are calculated for ESSP
could also solve SSP making some regions from the basis obsolete.

Signed-off-by: Uli Schlachter <uli.schlachter@informatik.uni-oldenburg.de>