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>

src/compiler/nir/nir.h

@@ -453,6 +453,12 @@ typedef struct nir_variable {
        */
       unsigned how_declared:2;
 
+      /**
+       * Is this variable per-view?  If so, we know it must be an array with
+       * size corresponding to the number of views.
+       */
+      unsigned per_view:1;
+
       /**
        * \brief Layout qualifier for gl_FragDepth.
        *