nir/opt_load_store_vectorize: fix check_for_robustness() with deref access

nir/opt_load_store_vectorize: fix check_for_robustness() with deref access

We could do better if we knew the nir_address_format to obtain
addition_bits, but the only affected driver (Turnip) probably won't
benefit because it doesn't vectorize across vec4.

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Fixes: 2e7bceb2204 ("nir/load_store_vectorizer: fix check_for_robustness() with indirect loads")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/4922
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11382>