nir/gcm: allow derivative dependent intrinisics to be moved earlier

nir/gcm: allow derivative dependent intrinisics to be moved earlier

We can't move them later as we could move them into non-uniform
control flow, but moving them earlier should be fine.

This helps avoid a bunch of spilling in unigine shaders due to
moving the tex instructions sources earlier (outside if branches)
but not the instruction itself.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4636>