Error: Input to Brillig function is in a separate subgraph to output

[jialinli98]

When we attempt to convert field element to U256 with u256::from(field). We will get the following error when running
nargo compile --package is_ape_owner --deny-warnings --skip-brillig-constraints-check

bug: Input to Brillig function is in a separate subgraph to output
    ┌─ /Users/jli/nargo/github.com/noir-lang/noir-bignum/v0.8.3/src/fns/constrained_ops.nr:490:18
    │
490 │         unsafe { __validate_gte_with_flags(lhs, rhs) };
    │                  ----------------------------------- There is no path from the output of this Brillig call to either return values or inputs of the circuit, which creates an independent subgraph. This is quite likely a soundness vulnerability
    │

I created a customized field conversion function to avoid this error temporarily.

fn field_to_u256(f: Field) -> U256 {
    let bytes = f.to_be_bytes();
    from(bytes)
}