freedreno/ir3/ra: add def/use iterators

freedreno/ir3/ra: add def/use iterators

Decouple the messy logic of figuring out vreg names defined/used by an
instruction from the logic of what to do about it by introducing
iterators.  There is still *some* array vs ssa special casing in
ra_block_compute_live_ranges(), but less than before.  And this will
avoid introducing a second copy of the def/use logic in a following
patch which uses the liveranges to calculate the maximum # of live
values (which is the optimal target for max physical register window
to round-robin within).

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4272>