Fix qasm output for bitmasked measurements (#4756)

Our invert mask only affects the stored value of the measurement, not the qubit itself. So in the qasm we need to un-invert the qubit after measurement.
quantumlib · Dec 14, 2021 · ce1a242 · ce1a242
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commit ce1a242
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diff --git a/cirq-core/cirq/circuits/qasm_output_test.py b/cirq-core/cirq/circuits/qasm_output_test.py
@@ -494,6 +494,7 @@ def filter_unpredictable_numbers(text):
 measure q[1] -> m_multi[0];
 x q[2];  // Invert the following measurement
 measure q[2] -> m_multi[1];
+x q[2];  // Undo the inversion
 measure q[3] -> m_multi[2];
 
 // Dummy operation

diff --git a/cirq-core/cirq/ops/measurement_gate.py b/cirq-core/cirq/ops/measurement_gate.py
@@ -181,6 +181,8 @@ def _qasm_(self, args: 'cirq.QasmArgs', qubits: Tuple['cirq.Qid', ...]) -> Optio
             if inv:
                 lines.append(args.format('x {0};  // Invert the following measurement\n', qubit))
             lines.append(args.format('measure {0} -> {1:meas}[{2}];\n', qubit, self.key, i))
+            if inv:
+                lines.append(args.format('x {0};  // Undo the inversion\n', qubit))
         return ''.join(lines)
 
     def _quil_(