nir/lower_samplers: split out _legacy version for deref chains

nir/lower_samplers: split out _legacy version for deref chains

To simplify the transition, and make things bisectable, split out a
legacy copy or lower_samplers.  This way the i965 and gallium drivers
can independently switch over to deref instructions.

Since the lower_samplers_as_deref pass is only used by gallium drivers,
it can be converted in lock-step with moving the lower_deref_instrs
pass, and so does not need a corresponding _legacy clone.

This legacy pass will be removed in a future commit.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Acked-by: Rob Clark <robdclark@gmail.com>
Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Acked-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>