Fixes disassembler error for mcx gate (Qiskit#6291)

* Fix disassemble of mcx gates

* Add compiler test

Co-authored-by: Kevin Krsulich <kevin.krsulich@ibm.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

qiskit/assembler/disassemble.py

@@ -126,6 +126,8 @@ def _experiments_to_circuits(qobj):
                     _inst = instr_method(params, qubits)
                 elif i.name == 'isometry':
                     _inst = instr_method(*params, qubits, clbits)
+                elif i.name in ['mcx', 'mcu1', 'mcp']:
+                    _inst = instr_method(*params, qubits[:-1], qubits[-1], *clbits)
                 else:
                     _inst = instr_method(*params, *qubits, *clbits)
             elif name == 'bfunc':

test/python/compiler/test_disassembler.py

@@ -189,6 +189,21 @@ def test_circuit_with_simple_conditional(self):
         self.assertEqual(circuits[0], qc)
         self.assertEqual({}, header)
 
+    def test_circuit_with_mcx(self):
+        """Verify disassemble handles mcx gate - #6271."""
+        qr = QuantumRegister(5)
+        cr = ClassicalRegister(5)
+        qc = QuantumCircuit(qr, cr)
+        qc.mcx([0, 1, 2], 4)
+        qobj = assemble(qc)
+        circuits, run_config_out, header = disassemble(qobj)
+        run_config_out = RunConfig(**run_config_out)
+        self.assertEqual(run_config_out.n_qubits, 5)
+        self.assertEqual(run_config_out.memory_slots, 5)
+        self.assertEqual(len(circuits), 1)
+        self.assertEqual(circuits[0], qc)
+        self.assertEqual({}, header)
+
     def test_multiple_conditionals_multiple_registers(self):
         """Verify disassemble handles multiple conditionals and registers."""
         qr = QuantumRegister(3)