target/riscv/vector_helper.c: skip set tail when vta is zero

The function is a no-op if 'vta' is zero but we're still doing a lot of
stuff in this function regardless. vext_set_elems_1s() will ignore every
single time (since vta is zero) and we just wasted time.

Skip it altogether in this case. Aside from the code simplification
there's a noticeable emulation performance gain by doing it. For a
regular C binary that does a vectors operation like this:

=======
 #define SZ 10000000

int main ()
{
  int *a = malloc (SZ * sizeof (int));
  int *b = malloc (SZ * sizeof (int));
  int *c = malloc (SZ * sizeof (int));

  for (int i = 0; i < SZ; i++)
    c[i] = a[i] + b[i];
  return c[SZ - 1];
}
=======

Emulating it with qemu-riscv64 and RVV takes ~0.3 sec:

$ time ~/work/qemu/build/qemu-riscv64 \
    -cpu rv64,debug=false,vext_spec=v1.0,v=true,vlen=128 ./foo.out

real    0m0.303s
user    0m0.281s
sys     0m0.023s

With this skip we take ~0.275 sec:

$ time ~/work/qemu/build/qemu-riscv64 \
    -cpu rv64,debug=false,vext_spec=v1.0,v=true,vlen=128 ./foo.out

real    0m0.274s
user    0m0.252s
sys     0m0.019s

This performance gain adds up fast when executing heavy benchmarks like
SPEC.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Message-Id: <20230427205708.246679-2-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/vector_helper.c

@@ -268,12 +268,17 @@ static void vext_set_tail_elems_1s(CPURISCVState *env, target_ulong vl,
                                    void *vd, uint32_t desc, uint32_t nf,
                                    uint32_t esz, uint32_t max_elems)
 {
-    uint32_t total_elems = vext_get_total_elems(env, desc, esz);
-    uint32_t vlenb = riscv_cpu_cfg(env)->vlen >> 3;
+    uint32_t total_elems, vlenb, registers_used;
     uint32_t vta = vext_vta(desc);
-    uint32_t registers_used;
     int k;
 
+    if (vta == 0) {
+        return;
+    }
+
+    total_elems = vext_get_total_elems(env, desc, esz);
+    vlenb = riscv_cpu_cfg(env)->vlen >> 3;
+
     for (k = 0; k < nf; ++k) {
         vext_set_elems_1s(vd, vta, (k * max_elems + vl) * esz,
                           (k * max_elems + max_elems) * esz);