target/riscv/vector_helper.c: use 'vlenb'

target/riscv/vector_helper.c: use 'vlenb'

Use 'cpu->cfg.vlenb' instead of 'cpu->cfg.vlen >> 3'.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240122161107.26737-8-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>