SynchronizeTerminalMeasurements doesn't handle multi-qubit measurements

[daxfohl]

Description of the issue

The logic for SynchronizeTerminalMeasurements goes qubit-by-qubit checking for measurements that are terminal but not at the last moment, and moves such measurements to the last moment. Thus if a two-qubit measurement has a single-qubit gate after it, the logic still moves the measurement to terminal position since one of its qubits is unblocked.

How to reproduce the issue

Add the following test to synchronize_terminal_measurements_test.py. The circuit should be unmodified by the transformer. But the transformer moves the measurement to the end.

def test_multi_qubit():
    q0, q1 = cirq.LineQubit.range(2)
    circuit = cirq.Circuit(
        cirq.measure(q0, q1, key='m'), cirq.H(q1)
    )
    assert_optimizes(before=circuit, after=circuit.copy())

Cirq version
0.14.1