isl_tab_pip: keep cache of partial solutions

isl_tab_pip: keep cache of partial solutions

This cache allows us to combine two partial solutions into a single
partial solution if they have the same mapping.  The domain of the
single partial solution is that before it was split to arrive at
the two domains of the two partial solutions.
In particular, this happens if it turns out there is no solution
in either part of the context.
It also happens when the initial set has existentially quantified
variables.  In this case, we only insist that these variables
have an integer value, but we don't care about the actual value.
The solutions in the two parts of the context may therefore still
agree on the remaining variables.