target/riscv: rvv-1.0: take fractional LMUL into vector max elements calculation

target/riscv: rvv-1.0: take fractional LMUL into vector max elements calculation

Update vext_get_vlmax() and MAXSZ() to take fractional LMUL into
calculation for RVV 1.0.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20211210075704.23951-27-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>