target/riscv: rvv-1.0: Simplify vfwredsum code

Remove duplicate code by wrapping vfwredsum_vs's OP function.

Signed-off-by: Yang Liu <liuyang22@iscas.ac.cn>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Message-Id: <20220817074802.20765-1-liuyang22@iscas.ac.cn>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>