nir: Add support for generic pointers

The way they're handled is that deref->modes is treated as a bitfield of
possible modes.  Variables are required to have a specific mode and
derefs with deref_type_var are as well.

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