glsl: Don't do structure splitting until link time.

We were splitting on each side of an unlinked program, and the two
sides lost track of which variables they referenced, resulting in
assertion failure during validation.  Fixes piglit
link-struct-uniform-usage.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/glsl_parser_extras.cpp

@@ -891,8 +891,8 @@ do_common_optimization(exec_list *ir, bool linked, unsigned max_unroll_iteration
    if (linked) {
       progress = do_function_inlining(ir) || progress;
       progress = do_dead_functions(ir) || progress;
+      progress = do_structure_splitting(ir) || progress;
    }
-   progress = do_structure_splitting(ir) || progress;
    progress = do_if_simplification(ir) || progress;
    progress = do_discard_simplification(ir) || progress;
    progress = do_copy_propagation(ir) || progress;