isl_map_dim_{min,max}: use isl_basic_map_lexopt_pw_multi_aff

isl_map_dim_{min,max}: use isl_basic_map_lexopt_pw_multi_aff

Parts of the isl_basic_map_lexopt_* implementations were merged in
isl-0.17.1-120-gd30117d6e1 (merge isl_tab_basic_map_partial_lexopt*
implementations, Tue Jun 14 10:23:30 2016 +0200) and
isl-0.17.1-122-g7c5c44c1e0 (merge isl_basic_*_partial_lex* implementations,
Tue Jun 14 16:15:41 2016 +0200), meaning that improvements such as
isl-0.17.1-128-g8590c53694 (isl_*_lexopt: use actual domain of input as
initial context, Tue Jun 14 13:12:04 2016 +0200) only had to be implemented
once.
The function isl_basic_map_foreach_lexopt, however, was not left out
of the merging.

Use isl_basic_map_lexopt_pw_multi_aff instead of isl_basic_map_foreach_lexopt
inside isl_map_dim_{min,max} to obtain more consistent results across
calls to different kinds of optimization functions.
Since this is the last remaining call to isl_basic_map_foreach_lexopt,
this also allows isl_basic_map_foreach_lexopt to be removed
in the next commit.

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