isl_tab_basic_set_non_trivial_lexmin: extract out fix_zero and pos_neg
isl_tab_basic_set_non_trivial_lexmin: extract out fix_zero and pos_neg

This reduces the size of isl_tab_basic_set_non_trivial_lexmin and
allows the side selection to be documented in more detail.
This in turn will help clarify the way this selection
is changed in an upcoming commit.

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

PollyLabs/isl,nicolasvasilache/isl,inducer/isl-mirror,PollyLabs/isl,nicolasvasilache/isl,nicolasvasilache/isl,tobig/isl,nicolasvasilache/isl,Meinersbur/isl,Meinersbur/isl,tobig/isl,PollyLabs/isl,inducer/isl-mirror,PollyLabs/isl,PollyLabs/isl,inducer/isl-mirror,inducer/isl-mirror,inducer/isl-mirror,tobig/isl,Meinersbur/isl,nicolasvasilache/isl,tobig/isl,Meinersbur/isl