Preserve max moment of control key while appending to circuit (#7626)

Control operations sharing the same control key could be assigned an
earlier moment index, causing later measurements to appear before
earlier controls.

This change updates `ckey_indices` to keep the maximum moment index seen
so far, preventing reordering of classical controls and measurements.

Fixes #7622, fixes #7623

---------

Co-authored-by: Pavol Juhas <juhas@google.com>

Author: p51lee

cirq-core/cirq/circuits/circuit.py

@@ -3002,13 +3002,15 @@ def get_earliest_accommodating_moment_index(
     mop_index = last_conflict + 1
 
     # Update our dicts with data from this `mop` placement. Note `mop_index` will always be greater
-    # than the existing value for all of these, by construction.
+    # than the existing value for qubits and measurement keys, by construction.
     for qubit in mop_qubits:
         qubit_indices[qubit] = mop_index
     for key in mop_mkeys:
         mkey_indices[key] = mop_index
+    # For control keys, keep the maximum moment index seen so far because ops with the same control
+    # keys can commute past each other.
     for key in mop_ckeys:
-        ckey_indices[key] = mop_index
+        ckey_indices[key] = max(mop_index, ckey_indices.get(key, -1))
 
     return mop_index
 

cirq-core/cirq/circuits/circuit_test.py

@@ -208,6 +208,24 @@ def test_append_control_key() -> None:
     assert len(c) == 1
 
 
+def test_append_control_key_before_measure() -> None:
+    c = cirq.Circuit()
+    q1, q2 = cirq.LineQubit.range(2)
+    c.append(cirq.X(q1))
+    c.append(cirq.X(q1))
+    c.append(cirq.X(q1).with_classical_controls('a'))
+    c.append(cirq.X(q2).with_classical_controls('a'))
+    c.append(cirq.measure(q2, key='a'))
+    assert c == cirq.Circuit(
+        [
+            cirq.Moment(cirq.X(q1), cirq.X(q2).with_classical_controls('a')),
+            cirq.Moment(cirq.X(q1)),
+            cirq.Moment(cirq.X(q1).with_classical_controls('a')),
+            cirq.Moment(cirq.measure(q2, key='a')),
+        ]
+    )
+
+
 def test_append_multiple() -> None:
     a = cirq.NamedQubit('a')
     b = cirq.NamedQubit('b')