freedreno/ir3: round-robin RA

freedreno/ir3: round-robin RA

In the second (scalar pass) use the information about # of registers
used in the first pass as the target max, and round-robin within that
range.  This generally gives the post-RA sched pass more opportunities
to re-order instructions to remove nop's.

Also, we can be a bit clever when assigning dest registers for SFU
instructions, by picking the register used for it's src (if available
and already assigned).  This avoids some (ss) syncs caused by write
after read hazards.  (Ie. the SFU instruction will read it's own src
before writing dest.)

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4071>