Pad non-idle qubits in control flow blocks

[dieris]

Summary

This PR proposes two changes:

• Reenable padding in qubits within control flow blocks.
• Omit idle qubits entirely from control flow blocks.

Details and comments

The first change is necessary to apply dynamical decoupling within the control flow blocks.
The second change is useful where a second padding / DD pass is required. This way, we would only keep the active qubits in the control blocks and prevent scheduling the idle ones.

Example

creg = ClassicalRegister(1)
qreg = QuantumRegister(3)
circ = QuantumCircuit(qreg,creg)
circ.sx(qreg[0])
circ.x(qreg[1])
circ.sx(qreg[2])
circ.barrier()
circ.measure(0,creg[0])
with circ.if_test((creg[0], 0)):
    circ.x(qreg[1])

pm = PassManager([ALAPScheduleAnalysis(durations), PadDynamicalDecoupling(durations, [XGate(), XGate()])])