isl_basic_map_lexopt*: preinitialize domain in the same way
When isl_basic_map_lexmin_pw_multi_aff was introduced in
6fb5454
(add isl_basic_map_lexmin_pw_multi_aff, Tue Aug 16 18:46:25 2011 +0200),
the way it constructed the domain was the same as in isl_basic_map_lexopt.
When the latter was changed in
a47882c (isl_basic_map_lexopt: preinitialize
domain, Tue Mar 12 17:23:49 2013 +0100), isl_basic_map_lexmin_pw_multi_aff
was not updated accordingly. Do so now.
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>