compiler: Add two new system values for subgroups

This will be required for SPIR-V subgroup support

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>

src/compiler/shader_enums.h

@@ -397,6 +397,14 @@ typedef enum
    SYSTEM_VALUE_SUBGROUP_LT_MASK,
    /*@}*/
 
+   /**
+    * Builtin variables added by VK_KHR_subgroups
+    */
+   /*@{*/
+   SYSTEM_VALUE_NUM_SUBGROUPS,
+   SYSTEM_VALUE_SUBGROUP_ID,
+   /*@}*/
+
    /*@}*/
 
    /**