nir: Make nir_copy_deref follow the "clone" pattern

nir: Make nir_copy_deref follow the "clone" pattern

We rename it to nir_deref_clone, re-order the sources to match the other
clone functions, and expose nir_deref_var_clone.  This past part, in
particular, lets us get rid of quite a few lines since we no longer have
to call nir_copy_deref and wrap it in deref_as_var.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>