target/riscv: optimize helper for vmv<nr>r.v

target/riscv: optimize helper for vmv<nr>r.v

LEN is not used for GEN_VEXT_VMV_WHOLE macro, so vmv<nr>r.v can share
the same helper

Signed-off-by: Weiwei Li <liweiwei@iscas.ac.cn>
Signed-off-by: Junqiang Wang <wangjunqiang@iscas.ac.cn>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20220325085902.29500-2-liweiwei@iscas.ac.cn>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>