Fix conditional barriers

[qciaran]

When I try to run the qasm with conditional barriers like if(m==0) barrier q[0], q[1]; using qasm_to_phir() it raises ValidationError.

It also might be useful to introduce a barrier into PHIR...

[qciaran]

Here is example Qasm:

OPENQASM 2.0;
include "qelib1.inc";

qreg q[2];
creg m[2];

h q[0];
if(m==0) barrier q[0], q[1];
CX q[0], q[1];

measure q -> m;

[qartik]

I believe the issue is because we are skipping barriers in PHIR, we do have a todo at

pytket-phir/pytket/phir/phirgen.py

Lines 216 to 219 in 427c7e0

	case tk.BarrierOp():
	# TODO(kartik): confirm with Ciaran/spec
	# https://github.com/CQCL/phir/blob/main/spec.md
	logger.debug("Skipping Barrier instruction")

:

        case tk.BarrierOp():
            # TODO(kartik): confirm with Ciaran/spec
            # https://github.com/CQCL/phir/blob/main/spec.md
            logger.debug("Skipping Barrier instruction")

I wasn't sure what the right behavior should be as the spec doesn't mention it. Could you suggest what instruction should be emitted? I can update both the spec and phirgen for this issue.

Here's the PHIR being generated right now:

{
    'format': 'PHIR/JSON',
    'version': '0.1.0',
    'metadata': {'source': 'pytket-phir v0.3.1.dev2'},
    'ops': [
        {'data': 'qvar_define', 'data_type': 'qubits', 'variable': 'q', 'size': 2},
        {'data': 'cvar_define', 'data_type': 'u32', 'variable': 'm', 'size': 2},
        {'//': 'H q[0];'},
        {'qop': 'H', 'angles': None, 'args': [['q', 0]]},
        {'//': 'IF ([m[0], m[1]] == 0) THEN IF ([m[0], m[1]] == 0) THEN Barrier q[0], q[1];'},
        {'block': 'if', 'condition': {'cop': '==', 'args': ['m', 0]}, 'true_branch': [None]},
        {'//': 'CX q[0], q[1];'},
        {'qop': 'CX', 'angles': None, 'args': [[['q', 0], ['q', 1]]]},
        {'//': 'Measure q[0] --> m[0];'},
        {'qop': 'Measure', 'returns': [['m', 0]], 'args': [['q', 0]]},
        {'//': 'Measure q[1] --> m[1];'},
        {'qop': 'Measure', 'returns': [['m', 1]], 'args': [['q', 1]]}
    ]
}

Another related issue #55