isl_pw_aff_on_shared_domain: perform gist on results wrt shared domains

isl_pw_aff_on_shared_domain: perform gist on results wrt shared domains

The shared domain may have extra constraints compared to the original
domains, which may help to simplify some of the integer divisions involved.
As a side effect, the gist operation may also simplify integer divisions
that could have been simplified even on a universe domain.
We should probably handle such cases much earlier during the construction
of isl_affs.

Reported-by: Tobias Grosser <tobias@grosser.es>
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>