freedreno/ir3: DCE unused arrays

freedreno/ir3: DCE unused arrays

Letting unused arrays stick around confuses RA, which assigns vreg names
to the unused arrays, but then does not precolor them (because they are
unused).  This leads to an assert in ra_select_reg_merged():

  skqp: ../src/freedreno/ir3/ir3_ra.c:589: name_to_instr: Assertion '!name_is_array(ctx, name)' failed.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3262
Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5907>