ppcg.c: compute_order_dependences: avoid use of before relation
The before relation contains all pairs of statement instances
where the first is executed before the second according to
the original schedule. Its size is therefore quadratic in
the number of statements.
The before relation computed in compute_live_range_reordering_dependences
is currently used in both compute_order_dependences and
compute_forced_dependences. Remove its use from compute_order_dependences
first, so that we can remove it completely in the next commit.
Do so by computing the order dependences directly from the original
schedule.
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>