RISC-V: Fix recursive vsetvli checking [PR114172]

RISC-V: Fix recursive vsetvli checking [PR114172]

extract_single_source will recursive checking the sources to
make sure if it's single source, however it may cause infinite
recursive when the source is come from itself, so it should just skip
first source to prevent that.

NOTE: This logic has existing on trunk/GCC 14, but it included in a big
vsetvli improvement patch, which is not backport to GCC 13.

```

void saxpy_rvv_m8(float *y, long vl)
{
    for (;;)
    {
        vl = __riscv_vsetvl_e32m8(vl); //ICE
        vfloat32m8_t y_vec;
        __riscv_vse32_v_f32m8(y, y_vec, vl);
    }
}
```

gcc/ChangeLog:

	PR target/114172
	* config/riscv/riscv-vsetvl.cc (extract_single_source):
	Skip first set.

gcc/testsuite/ChangeLog:

	PR target/114172
	* gcc.target/riscv/rvv/vsetvl/pr114172.c: New.