isl_ast_codegen.c: generate_domain: intersect with simple hull of build domain

isl_ast_codegen.c: generate_domain: intersect with simple hull of build domain

We used to intersect with the build domain itself, but this domain
may be a disjunction and intersecting with it may result in a lot of
extra conditions because it is more difficult to simplify away
a disjunction.  At least, isl's gist is not very good at simplifying
away disjunctions at the moment.
Note that by approximating the build domain, we may lose some special
cases where we would detect single-valuedness,
but we can always recover from that through the nested call.

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