nir/lower_io: Add support for lowering deref_mode_is

The guts are still missing so it will blow up if it sees any
deref_mode_is intrinsic that it can't constant-fold from the mode.

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>
3 files changed