isl_tab_basic_set_non_trivial_lexmin: extract out better_next_side
isl_tab_basic_set_non_trivial_lexmin: extract out better_next_side

This reduces the size of isl_tab_basic_set_non_trivial_lexmin.

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

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