add option to bound the constant scheduling coefficients

add option to bound the constant scheduling coefficients

If larger coefficients appear as part of the input dependences, the
schedule calculation can take a very long time. We observed that the
main overhead in this calculation is due to optimizing the constant
coefficients. They are misused to increase locality by merging several
unrelated dimensions into a single dimension. This unwanted optimization
increases the complexity of the code and slows down the generated code.

We introduce a new option that bounds the values in the constant
dimension by a user defined value. If the right value is choosen,
costly overoptimization can be prevented.

This solution works, but requires the user to specify the value by
which the constants are bound. For the moment, this is our best
solution, but we hope to to find a more generic one later on.

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