freedreno/ir3: Don't lower_gs multiple times

At least with gallium, this can be called multiple times via
pipe_screen::finalize_nir().  But it is not designed to be called
multiple times, and can result in vertex_flags getting 'optimized'
away.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/6720
Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17341>

src/freedreno/ir3/ir3_nir_lower_tess.c

@@ -909,6 +909,11 @@ ir3_nir_lower_gs(nir_shader *shader)
 {
    struct state state = {};
 
+   /* Don't lower multiple times: */
+   nir_foreach_shader_out_variable (var, shader)
+      if (var->data.location == VARYING_SLOT_GS_VERTEX_FLAGS_IR3)
+         return;
+
    if (shader_debug_enabled(shader->info.stage)) {
       mesa_logi("NIR (before gs lowering):");
       nir_log_shaderi(shader);