target/riscv: rvv: Fix Zvfhmin checking for vfwcvt.f.f.v and vfncvt.f.f.w instructions

target/riscv: rvv: Fix Zvfhmin checking for vfwcvt.f.f.v and vfncvt.f.f.w instructions

According v spec 18.4, only the vfwcvt.f.f.v and vfncvt.f.f.w
instructions will be affected by Zvfhmin extension.
And the vfwcvt.f.f.v and vfncvt.f.f.w instructions only support the
conversions of

* From 1*SEW(16/32) to 2*SEW(32/64)
* From 2*SEW(32/64) to 1*SEW(16/32)

Signed-off-by: Max Chou <max.chou@sifive.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Cc: qemu-stable <qemu-stable@nongnu.org>
Message-ID: <20240322092600.1198921-2-max.chou@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>