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>

target/riscv/vector_helper.c

@@ -4659,7 +4659,9 @@ void HELPER(NAME)(void *vd, void *v0, void *vs1,          \
         }                                                 \
         s1 = OP(s1, (TD)s2);                              \
     }                                                     \
-    *((TD *)vd + HD(0)) = s1;                             \
+    if (vl > 0) {                                         \
+        *((TD *)vd + HD(0)) = s1;                         \
+    }                                                     \
     env->vstart = 0;                                      \
     /* set tail elements to 1s */                         \
     vext_set_elems_1s(vd, vta, esz, vlenb);               \
@@ -4745,7 +4747,9 @@ void HELPER(NAME)(void *vd, void *v0, void *vs1,           \
         }                                                  \
         s1 = OP(s1, (TD)s2, &env->fp_status);              \
     }                                                      \
-    *((TD *)vd + HD(0)) = s1;                              \
+    if (vl > 0) {                                          \
+        *((TD *)vd + HD(0)) = s1;                          \
+    }                                                      \
     env->vstart = 0;                                       \
     /* set tail elements to 1s */                          \
     vext_set_elems_1s(vd, vta, esz, vlenb);                \