radv: call nir_copy_prop() after load-store vectorization
The load-store vectorizer can create a large amount of unnecessary nir_op_vec and nir_op_mov instructions. This prevents nir_opt_move from stalling to much and potentially also helps other passes.
Closes: #4778 (closed) Fixes: 1958381c ('radv: Reorder some NIR optimizations in preparation for the I/O changes.')