glsl/shader_enums: Add an enum for Vulkan InstanceIndex

In Vulkan, you have InstanceIndex which begins at the base instance value
rather than the zero-based InstanceID of GL.

Reviewed-by: Rob Clark <robdclark@gmail.com>

src/compiler/shader_enums.h

@@ -378,6 +378,13 @@ typedef enum
     */
    SYSTEM_VALUE_INSTANCE_ID,
 
+   /**
+    * Vulkan InstanceIndex.
+    *
+    * InstanceIndex = gl_InstanceID + gl_BaseInstance
+    */
+   SYSTEM_VALUE_INSTANCE_INDEX,
+
    /**
     * DirectX-style vertex ID.
     *