freedreno/ir3: track "keeps" in ir

freedreno/ir3: track "keeps" in ir

Previously we had a fixed array to track kills, since they don't
generate an SSA value, and then cheated by stuffing them in the
outputs array before sending things through depth/sched/etc.  But
store instructions will need similar treatment.  So convert this
over to a more general array of instructions that must be kept
and fix up the places that were previously relying on kills being
in the output array.

Signed-off-by: Rob Clark <robclark@freedesktop.org>