nir_to_tgsi: Do the required cleanup for nir_opt_find_array_copies().

nir_to_tgsi: Do the required cleanup for nir_opt_find_array_copies().

If we made a copy deref, then we need to do dead-write elimination for the
pervious writes or we'll just emit the same copy deref again next time
around.  And, at the end of the opt loop, we need to lower copy derefs
because later passes (locals_to_regs, notably) depend on it.

Fixes infinite opt loop on fs-function-inout-array with virgl on NTT.

Reviewed-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15899>