nir: Filter modes of scoped memory barrier in nir_opt_load_store_vectorize

Otherwise a scoped memory barrier containing nir_var_mem_ubo (which
memoryBarrier() does lower to) would incorrectly prevent the
optimization to happen in UBOs.

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5980>