target/riscv: rvv: Fix unexpected behavior of vector reduction instructions when...

target/riscv: rvv: Fix unexpected behavior of vector reduction instructions when vl is 0

According to the Vector Reduction Operations section in the RISC-V "V"
Vector Extension spec,
"If vl=0, no operation is performed and the destination register is not
updated."

The vd should be updated when vl is larger than 0.

Fixes: fe5c9ab1fc ("target/riscv: vector single-width integer reduction instructions")
Fixes: f714361ed7 ("target/riscv: rvv-1.0: implement vstart CSR")
Signed-off-by: Max Chou <max.chou@sifive.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20250124101452.2519171-1-max.chou@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
(cherry picked from commit ffd455963f230c7dc04965609d6675da687a5a78)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>