isl_tab_sample: perform greedy search before performing basis reduction

isl_tab_sample: perform greedy search before performing basis reduction

While basis reduction is very useful on difficult problems, it is
overkill on easy problems.  In fact, the basis reduction computation
can be quite expensive, especially in the presence of large coefficients.
This may happen in particular during the affine hull computation
when only a few sample points have been found so far.  If these
points have large values, then the equalities that describe their
affine hull may have coefficients with larger values still.

Perform a greedy search before computing a reduced basis.
We also perform a greedy search after computing the reduced basis
just in case the problem has become easy through this basis reduction.

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>