r600/sb: bail out if prepare_alu_group() doesn't find a proper scheduling

It is possible that the optimizer ends up in an infinite loop in
post_scheduler::schedule_alu(), because post_scheduler::prepare_alu_group()
does not find a proper scheduling. This can be deducted from
pending.count() being larger than zero and not getting smaller.

This patch works around this problem by signalling this failure so that the
optimizers bails out and the un-optimized shader is used.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103142


Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Gert Wollny <gw.fossdev@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit 69eee511)