target/riscv/vector_helper.c: use vlenb in HELPER(vsetvl)

target/riscv/vector_helper.c: use vlenb in HELPER(vsetvl)

Use the new 'vlenb' CPU config to validate fractional LMUL. The original
comparison is done with 'vlen' and 'sew', both in bits. Adjust the shift
to use vlenb.

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: <20240122161107.26737-9-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>