extract out shared isl_basic_map_output_defining_equality

extract out shared isl_basic_map_output_defining_equality

isl_pw_multi_aff_from_map calls isl_basic_map_plain_is_single_valued
to check if there is a defining equality for each output dimension and,
if so, it calls extract_isl_aff_from_basic_map to extract an isl_aff
from these defining equalities.
Both functions check for these equalities in the same way.
Extract out the check into a separate function.

This reduces code duplication and also allows us to more easily
change the kind of constraints that we consider to be defining equalities.

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