`AbstractCircuit.freeze` should not reallocate moments

[maffoo]

Description of the issue

When freezing circuits, Moment instances should be reused since they are immutable. This was previously the case, but seems to have changed recently (possibly due to #5332).

How to reproduce the issue

In [1]: q = cirq.q(0)

In [2]: c = cirq.Circuit(cirq.X(q), cirq.measure(q))

In [3]: f = c.freeze()

In [4]: [mc is fc for mc, fc in zip(c, f)]
Out[4]: [False, False]

Cirq version

1.1.0.dev20220801180412

[daxfohl]

Yes, looks like it was from #5332. The with_operations below is what does it. Moment.with_operations creates a new Moment even if the input list is empty.

Cirq/cirq-core/cirq/circuits/circuit.py

Line 1823 in 65ebb4a

self._moments.append(moments_by_index[i].with_operations(op_lists_by_index[i]))

You could either change the code here to skip the with_operations call if there are no operations to append, or alternatively, have Moment.with_operations return self if the input is empty.

[verult]

From Cirq sync - this will be considered a bug fix as it returns the API behavior to the original intention.