intel/compiler: Add a uses_firstvertex flag

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/intel/compiler/brw_compiler.h

@@ -978,6 +978,7 @@ struct brw_vs_prog_data {
    bool uses_vertexid;
    bool uses_instanceid;
    bool uses_basevertex;
+   bool uses_firstvertex;
    bool uses_baseinstance;
    bool uses_drawid;
 };