nir: Add a deref instruction type

nir: Add a deref instruction type

This commit adds a new instruction type to NIR for handling derefs.
Nothing uses it yet but this adds the data structure as well as all of
the code to validate, print, clone, and [de]serialize them.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Acked-by: Rob Clark <robdclark@gmail.com>
Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Acked-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>