nir/sink,nir/move: move/sink nir_op_mov

nir/sink,nir/move: move/sink nir_op_mov

Can uncover opportunities to move other instructions. This can increase
register usage, but that doesn't seem to actually happen.

This optimizes a pattern of a load_per_vertex_input followed by several
moves and then a store_output in a different block.

v2: add nir_move_copies to make it optional

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Acked-by: Jason Ekstrand <jason@jlekstrand.net> (v1)
Acked-by: Rob Clark <robdclark@chromium.org>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/2420>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/2420>