target/riscv: Refactor vector-vector translation macro

Refactor the non SEW-specific stuff out of `GEN_OPIVV_TRANS` into
function `opivv_trans` (similar to `opivi_trans`). `opivv_trans` will be
used in proceeding vector-crypto commits.

Signed-off-by: Kiran Ostrolenk <kiran.ostrolenk@codethink.co.uk>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Signed-off-by: Max Chou <max.chou@sifive.com>
Message-ID: <20230711165917.2629866-3-max.chou@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>