cloog_loop_simplify: split up union-domains before simplifying

cloog_loop_simplify: split up union-domains before simplifying

The clast construction assumes that none of the loop domains
involve any unions.  To ensure this property holds an additional
call to cloog_loop_disjoint was performed after the actual simplification.
This interacts badly with the save_domains option.
If cloog_loop_disjoint actually performs any operation, then
the unsimplified domain attached is discarded.

Another possible drawback of the old approach is that if a union domain
ever appears on a node that is not a leaf node, then that union domain
will be used as the context of a simplification operation, making
it less effective then had the domain been split first.
(It's not immediately obvious if this can ever happen.)

The simplification process itself never introduces any union,
so it is safe to perform the split (if needed) before the simplification.
In particular, we now perform the split after the attempt to combine
union domains, but before the actual simplification.

The comment is also changed as it has been outdated.
In particular, the union that is present in the test/iftest2.cloog
input has now already disappeared before we reach this stage.

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>