mesa: clear shader_info::is_lowered in prog_to_nir

This needs to be resetted each time prog_to_nir is called because it
turns st_nir_assign_vs_in_locations into a no-op when set.

Fixes: 81d106d6ec ("radeonsi: lower IO intrinsics - complete rewrite of input/output scanning")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/5001
Reviewed-by: Isaac Bosompem <mrisaacb@google.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11636>

src/mesa/program/prog_to_nir.c

@@ -1024,6 +1024,7 @@ prog_to_nir(const struct gl_program *prog,
    s->info.clip_distance_array_size = 0;
    s->info.cull_distance_array_size = 0;
    s->info.separate_shader = false;
+   s->info.io_lowered = false;
 
 fail:
    if (c->error) {