target/riscv: rvv-1.0: register gather instructions

target/riscv: rvv-1.0: register gather instructions

* Add vrgatherei16.vv instruction.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20211210075704.23951-36-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>