isl_map_simplify.c: eliminate_unit_divs: extract out eliminate_unit_div
isl_map_simplify.c: eliminate_unit_divs: extract out eliminate_unit_div

The function eliminate_unit_divs is fairly long and will be further
extended in the next commit.  Extract out part of the body
to make it a bit shorter.

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

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl