nir/remove_dead_variables: Properly handle deref casts

nir/remove_dead_variables: Properly handle deref casts

We already detect any incomplete deref chains (where the deref is used
for something other than another deref or a load/store) and flag the
variable as used thanks to deref_used_for_not_store.  All that's left to
do is to properly skip casts when cleaning up.

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