Brillig function calls with all constant inputs result in false positives for SSA connectedness check

[TomAFrench]

Consider the program

fn main() {
    assert_eq(foo(5), 5);
}

unconstrained fn foo(_x: Field) -> Field {
    5
}

This currently compiles down to an empty circuit as during ACIR generation we determine that the call to foo has constant arguments so we replace it with the expected output 5. However as of the end of the SSA passes (and so when @Rumata888's #5402 runs) the program looks like the below

acir(inline) fn main f0 {
  b0():
    v7 = call f1(Field 5)
    constrain v7 == Field 5
    return 
}
brillig fn foo f1 {
  b0(v0: Field):
    return Field 5
}

We can't quick fix this by just considering that brillig functions which have all constant inputs are connected to their outputs as the function could include foreign calls, etc. However we should be able to attempt execution of this brillig call in SSA similarly to how we do during ACIR generation.