scheduler: allow to bound the coefficients in the calculated schedule

scheduler: allow to bound the coefficients in the calculated schedule

isl already supports the bounding of the constant term of the calculated
schedule, but within polybench there are several cases where bounding
the coefficients of parameter and variable dimensions is also needed.

With a bound of 20 Polly can run the isl scheduler on all polybench 2.0 kernels
and takes for each less than 5 seconds, whereas without the bound five test
cases had scheduling times larger than 40 seconds. This is still slower, than
scheduling with gist simplified dependences but it yields better results.
The bounded, non-gist-simplified schedule yields for dynprog, lu, reg_detect
and trmm to faster code compared to scheduling with gist simplified
dependences. For gramschmidt it yields slower code.

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