radeonsi: Only call nir_lower_var_copies at the end of the opt loop

In 283ad859, radeonsi started using nir_find_var_copies. However, it was also calling nir_lower_var_copies in the optimization loop and the two can end up fighting. The simple solution is to wait to lower copies until the end of the optimization loop.

Fixes: 283ad859 "radeonsi: call nir_split_array_vars/shrink_vec..." Closes: #3550 (closed) Tested-by: Matt Coffin mcoffin13@gmail.com