kill (and other cat0/flow instructions) do not have a dst register.
Which was mostly harmless before, other than RA thinking it would need
a free register to write.  (But nothing consumed it, so the value would
be immediately dead.)  But this would cause more problems with postsched
which would see a bogus dependency.

Also, post-RA sched *does* need to see the dependency on the predicate
register.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <mesa/mesa!3569>