OptimizeSwapBeforeMeasure removes classically conditioned SwapGate

[kdk]

Information

• Qiskit Terra version:
• Python version:
• Operating system:

What is the current behavior?

From https://travis-ci.com/github/Qiskit/qiskit-terra/jobs/495770351#L997:

>>> qc = qk.QuantumCircuit.from_qasm_str('''
OPENQASM 2.0;
include "qelib1.inc";
qreg q3134[2];
creg c137[1];
cx q3134[1],q3134[0];
h q3134[1];
if(c137==1) swap q3134[1],q3134[0];
measure q3134[1] -> c137[0];
''')
>>> qc.draw()
         ┌───┐               
q3134_0: ┤ X ├────────X──────
         └─┬─┘┌───┐   │   ┌─┐
q3134_1: ──■──┤ H ├───X───┤M├
              └───┘┌──╨──┐└╥┘
 c137: 1/══════════╡ = 1 ╞═╩═
                   └─────┘ 0 
>>> qk.transpile(qc, optimization_level=3).draw()
         ┌───┐     ┌─┐
q3134_0: ┤ X ├─────┤M├
         └─┬─┘┌───┐└╥┘
q3134_1: ──■──┤ H ├─╫─
              └───┘ ║ 
 c137: 1/═══════════╩═
                    0 

Steps to reproduce the problem

What is the expected behavior?

Suggested solutions