nir,treewide: split lower_x_to_y into lower_x and has_y

This should produce the same result for all drivers, but it's more obvious when e.g. bfe or bfm can be created.

In the future it may even be possible to drop all but the general lowering for bitfield_insert and bitfield_extract and rely on optimizations to create bfe, bfm, bfi and bitfield_select.