freedreno/ir3: use SSA flag on dest register too

freedreno/ir3: use SSA flag on dest register too

We did this in some places before, but not consistantly.  But it will be
useful for two-pass RA, to identify which registers have already been
assigned.

While we are cleaning this up, use __ssa_src() and new __ssa_dst()
helper more consistently.  (If nothing else, this reduces the # of
callers of ir3_reg_create() to audit that we didn't miss something)

Signed-off-by: Rob Clark <robdclark@chromium.org>