freedreno/ir3: set (ss) on last_input if ldlv

freedreno/ir3: set (ss) on last_input if ldlv

It seems like (ei) handling doesn't sync on (ss), so we could end up in
a situation where we release varying storage before an ldlv for flat
shaded varyings completes.  Keep track if we've done an (ss) since the
last ldlv, and if not add (ss) flag to last_input which gets (ei).

Noticed with dEQP-GLES3.functional.fragment_out.random.24 and
dEQP-GLES3.functional.fragment_out.random.27, which previously passed by
luck because ir3_sched ordered instructions in a way that resulted in a
lucky (ss).

Signed-off-by: Rob Clark <robdclark@chromium.org>
Acked-by: Eric Anholt <eric@anholt.net>