isl_access_info_compute_flow: sort accesses in textual order

isl_access_info_compute_flow: sort accesses in textual order

Before, we tried to be clever and sorted the accesses in order
of increasing number of shared levels with the sink access.
However, this has little advantage over textual order.
Textually later accesses that cannot precede the sink at a given level
are simply skipped.
On the other hand, when we consider a loop level, we usually
do want to consider the textually last access first and not some
textually earlier access that happens to share more levels with the sink.
In particular, if this textually last access covers all the pending
sink accesses, there is no need to consider earlier accesses,
whereas you would always have to consider later accesses if you started
out with an earlier access.

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