isl_aff.c: pw_multi_aff_from_map_check_div: move div detection down

isl_aff.c: pw_multi_aff_from_map_check_div: move div detection down

In particular, move the detection of a pair of constraints
that define an integer division into isl_basic_map_extract_output_div,
renaming it to isl_basic_map_try_find_output_div and letting it
return an optional isl_aff.
The resulting isl_basic_map_try_find_output_div behaves similarly
to the isl_basic_map_try_find_output_mod that will be introduced
in the next commit, making it easier to call them in succession.

Signed-off-by: Sven Verdoolaege <sven@cerebras.net>