mesa: Replace string comparisons with SYSTEM_VALUE enum checks.

This is more efficient.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/main/shader_query.cpp

@@ -92,8 +92,8 @@ is_active_attrib(const ir_variable *var)
        * are enumerated, including the special built-in inputs gl_VertexID
        * and gl_InstanceID."
        */
-      return !strcmp(var->name, "gl_VertexID") ||
-             !strcmp(var->name, "gl_InstanceID");
+      return var->data.location == SYSTEM_VALUE_VERTEX_ID ||
+             var->data.location == SYSTEM_VALUE_INSTANCE_ID;
 
    default:
       return false;