nir: add streams to nir data

This will be used by gallium drivers.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/compiler/nir/nir.h

@@ -274,6 +274,14 @@ typedef struct nir_variable {
        */
       unsigned int driver_location;
 
+      /**
+       * Vertex stream output identifier.
+       *
+       * For packed outputs, bit 31 is set and bits [2*i+1,2*i] indicate the
+       * stream of the i-th component.
+       */
+      unsigned stream;
+
       /**
        * output index for dual source blending.
        */