add isl_pw_multi_aff_involves_dims
This function was presumable left out in
isl-0.07-202-g6f87ceab1c
(add isl_pw_multi_aff, Thu Sep 1 11:58:50 2011 +0200), because
it requires an isl_multi_aff_involves_dims, which was not available
at the time. However, the latter has since been added in
isl-0.12.1-190-g36f6eccc12 (add isl_multi_*_involves_dims,
Fri Dec 27 17:44:02 2013 +0100) so there is no longer any point
in making a special case for isl_pw_multi_aff.
Signed-off-by: Sven Verdoolaege <sven.verdoolaege@gmail.com>