Refactor IonTrapTwoQubitPass to use visitor pattern

[1tnguyen]

Using visitor pattern to implement the CNOT -> XX decomposition in IonTrapTwoQubitPass IRTransformation
to improve code readability and maintainability.

e.g., decompose 2-qubit gates into CNOT gates then revisit, etc.

[stevescia]

Hi, i'd would like to work on this issue as an assignee ; i have started the coding using the gitpod environment, following the vistor pattern; i'm currently looking at how totest the new code..

[1tnguyen]

Thanks @stevescia. I've assigned this to you. There is an existing test for this feature.

[stevescia]

Thien, I have the basics of the visitor pattern implemented and the unittests pass.

I am seeing that the cnot visitor is being triggered by the "XX" instruction, leading to additional instructions being added to the composite.. not sure if this is a bug, or an expected side offect of the visit instructions inside AllGateVisitor.hpp for the xx gate. please advise if something can be impl'd to avoid this trigger.

i worked around the above with a name check, in the CNOT visitor like:

    if (currInst->name() != std::string("CNOT"))
    {
        // found "XX" name; don't continue
        return;
    }
    
    // cnot visitor code continues