scheduler: replace split_parallel by split_scaled option

scheduler: replace split_parallel by split_scaled option

The split_parallel option was ill-conceived.
The condition split_parallel looked for was too specific
and the implementation was wrong.  Part of this has been fixed
in 1b2fdb6 (schedule.c: split_parallel: avoid invalid memory accesses,
Mon Jan 30 15:23:18 2012 +0100), but the code could in principle
still produce an incorrect schedule because it didn't check the sizes
of the constant terms.

The new option essentially applies strip-mining and should therefore
be safe.  In particular, it is applied when the linear parts of the
schedules have a non-trivial common divisor.  The strip-mining is
then performed with respect to this common divisor.

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