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

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

Add the following instructions:

* vaaddu.vv
* vaaddu.vx
* vasubu.vv
* vasubu.vx

Remove the following instructions:

* vadd.vi

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20211210075704.23951-42-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>