nir/sink: Don't sink load_ubo to outside of its defining loop

Previously, this could have made the resource divergent in code like
that which is genereated by nir_lower_non_uniform_access.

Fixes: da8ed68a ('nir: replace nir_move_load_const() with nir_opt_sink()')
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
(cherry picked from commit 5ac32b29)