target/riscv: rvv-1.0: allow load element with sign-extended

target/riscv: rvv-1.0: allow load element with sign-extended

For some vector instructions (e.g. vmv.s.x), the element is loaded with
sign-extended.

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: <20211210075704.23951-35-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>