Expand cases for generating a witness in prepare inputs

[vezenovm]

Related issue(s)

Resolves issue with not being able to use inputs to gadgets that had been set through a conditional statement.

Description

While writing the account circuit I was performing this conditional:

let mut signer: [Field; 2] = [signing_pub_key[0], signing_pub_key[1]];
    if create == 1 {
        signer[0] = account_public_key[0];
        signer[1] = account_public_key[1];
    }

Which was later used by this intrinsic gadget:

 let sig_res = std::schnorr::verify_signature(signer[0], signer[1], signature, message_byte_array);

When compiling this would lead to the error below:

We require that inputs to intrinsic gadgets be transformed into witnesses, however, we were not handling the case where the inputs to the function are a result of a conditional instruction. The result of this conditional does not have a witness associated with it.

Summary of changes

I simply added extra logic to prepare_inputs for cases other than a NodeObj::Obj. In the case that the NodeId is in the arithmetic cache, but does not yet have a witness generate, we simply generate a new witness for this instruction with generate_witness.

Dependency additions / changes

(If applicable.)

Test additions / changes

I added a little extra logic to our scalar_mul test. If you run nargo compile on this test using a nargo build from the current master you should see the error that I mentioned above. The test passes as expected with the current changes. You can also change the input of a for the scalar_mul test to see how the inputs to std::fixed_based::scalar_mul are changed.

Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt with default settings.
• I have linked this PR to the issue(s) that it resolves.
• I have reviewed the changes on GitHub, line by line.
• I have ensured all changes are covered in the description.

Additional context

(If applicable.)