legalize_block() can get run multiple times, which I didn't notice when
adding fine derivs support.  Other instruction clones change things such
that the legalization won't trigger again, but that didn't apply to the
DS.PP legalization.  To keep someone else from tripping over this, split
the one-shot legalization out of the iterative sync flag application.

Fixes failures in dEQP-VK.glsl.derivate.dfdxfine.*

Closes: mesa/mesa#3198
Part-of: <mesa/mesa!5699>