target/riscv: rvv-1.0: floating-point slide instructions

Add the following instructions:

* vfslide1up.vf
* vfslide1down.vf

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20211210075704.23951-52-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>