add support for unrolling
The user can specify which loop level to unroll. The loops
at the specified level, along with all inner loops will be unrolled,
provided they have a single lower bound and a number of iterations
that is bounded by a constant.
Other backends will have to implement their own
cloog_domain_can_unroll and cloog_domain_fixed_offset.
If cloog_domain_can_unroll always returns 0, then
cloog_domain_fixed_offset will never be called.
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>