nir: Add per_view attribute to nir_variable

nir: Add per_view attribute to nir_variable

If a nir_variable is tagged with per_view, it must be an array with
size corresponding to the number of views.  For slot-tracking, it is
considered to take just the slot for a single element -- drivers will
take care of expanding this appropriately.

This will be used to implement the ability of having per-view position
in a vertex shader in Intel platforms.

Acked-by: Rafael Antognolli <rafael.antognolli@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/2313>