nir/lower_io_to_vector: don't create arrays when not needed

Some backends require that there are no array varyings.

If there were no arrays in the input shader, the pass shouldn't have to
create new ones.

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2103
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2167


Fixes: bcd14756 ('nir/lower_io_to_vector: add flat mode')
Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
(cherry picked from commit 5404b7aa)