nir: replace nir_move_load_const() with nir_opt_sink() and combine nir_move_load_ubo() and nir_move_comparisons()

nir_opt_sink() is mostly the same as nir_move_load_const() but can also move undef instructions, load_ubo/load_interpolated_input/load_input (being careful with loops) and comparisons. This is used by ACO to reduce register pressure and move comparisons closer to their users.