pan/bi: Move pan_lower_sample_pos to next block

This should only run on frag shaders, let's group it the same way we
have it in midgard compiler.

Signed-off-by: Mary Guillemard <mary.guillemard@collabora.com>
Reviewed-by: Olivia Lee <olivia.lee@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36776>

src/panfrost/compiler/bifrost_compile.c

@@ -5995,11 +5995,10 @@ bifrost_preprocess_nir(nir_shader *nir, unsigned gpu_id)
    };
    NIR_PASS(_, nir, nir_lower_ssbo, &ssbo_opts);
 
-   NIR_PASS(_, nir, pan_lower_sample_pos);
-
    if (nir->info.stage == MESA_SHADER_FRAGMENT) {
       NIR_PASS(_, nir, nir_lower_is_helper_invocation);
       NIR_PASS(_, nir, pan_lower_helper_invocation);
+      NIR_PASS(_, nir, pan_lower_sample_pos);
    }
 
    /*