freedreno/ir3: fix mad copy propagation special case

freedreno/ir3: fix mad copy propagation special case

After the next patch enabling copy propagation for relative sources,
we'll need to dereference the n'th src in valid_flags(), so we actually
need to swap the sources before calling valid_flags().

But the logic was already a bit cumbersome, so move it into a helper
function.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Eric Anholt <eric@anholt.net>