isl_tab_basic_map_partial_lexopt: improve detection of equal partial solutions

isl_tab_basic_map_partial_lexopt: improve detection of equal partial solutions

The partial solution stack introduced in 4fff507 (isl_tab_pip: keep cache of
partial solutions, Fri Oct 16 14:46:28 2009 +0200) allows identical
partial solutions to be combined on their parent domain.
There are however cases where the partial solutions are not identical
but still equal to each other on one of the partial domains.
In this case the solution on the other partial domain is valid for both
partial domains and can also be used on the parent domain.

The test for such equal solutions is a bit more expensive than
the one that checks for identical solutions, but it should be
easier to detect opportunities for combining partial solutions
at this stage than it would be to detect them afterwards.

By combining partial solutions in this way, the result of
isl_pw_multi_aff_from_map can be improved in some cases,
as, e.g., in the new AST generation test case, derived from
a feature request by Tobias Grosser.

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