nir/opt_deref: Add an optimization for deref_mode_is

If opt_restrict_deref_modes makes progress, we may be able to figure out
the mode well enough to turn a deref_mode_is intrinsic into a constant.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6332>