nir: Don't lower the local work group size if it's variable.

nir: Don't lower the local work group size if it's variable.

If the local work group size is variable it won't be available
at compile time so we can't lower it in nir_lower_system_values().

Signed-off-by: Plamena Manolova <plamena.n.manolova@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Karol Herbst <kherbst@redhat.com>