isl_printer_print_union_flow: consistently print full dependences

isl_printer_print_union_flow: consistently print full dependences

Originally, an isl_union_flow would only keep track of
pairs of source and sink instances and
isl_printer_print_union_flow would print such dependence relations.
Since 5a519f702a (isl_union_flow: keep track of accessed elements,
Fri Nov 20 10:37:57 2015 +0100), isl_union_flow also keeps track
of the accessed element and isl_printer_print_union_flow was not
adjusted accordingly.  In particular, for the must dependences,
it would simply read the data field and include the accessed element,
while for the may dependences, it would call
isl_union_flow_get_may_dependence, which removes the accessed element.

Also include the accessed element in the may dependences
for consistency with the must dependences.

Signed-off-by: Sven Verdoolaege <sven.verdoolaege@gmail.com>