target/riscv: rvv-1.0: single-width saturating add and subtract instructions

target/riscv: rvv-1.0: single-width saturating add and subtract instructions

Sign-extend vsaddu.vi immediate value.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20211210075704.23951-47-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>