assert_eq always pass for U256

[xunilrj]

Doing a quick test here I found that this test pass.

#[test]
fn test_pow_u256() {
    let five = U256::from((0, 0, 0, 5));
    let two = U256::from((0, 0, 0, 2));
    assert_eq(five, two);
}

Maybe it is the same problem as #4868 because I am seeing fn eq of U256 body empty at compile_fn. This is returning a const bool undef and wreaking havoc from there.

Originally posted by @xunilrj in #4900 (comment)