nir: Move propagation of cast derefs to a new nir_opt_deref pass

nir: Move propagation of cast derefs to a new nir_opt_deref pass

We're going to want to do more deref optimizations going forward and
this gives us a central place to do them.  Also, cast propagation will
get a bit more complicated with the addition of ptr_as_array derefs.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>