nir: Don't bail too early in lower_mem_constant_vars

If there were no constant variables, we would bail out entirely.
However, we may still have constant input pointers coming in from the
client.

Fixes: 4360a8a2b3fce "nir/lower_io: Add support for nir_var_mem_constant"
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6472>
1 file changed