Promote CONTROL_FLOW_OP_NAMES to public API (#13057)

This is generally useful for Qiskit's built-in transpiler passes, and
we've had some interest in using it for the same reasons from outside
the library.  While the control-flow structure from within Rust might
change in the future, in the immediate term and from Python space, it
feels fair to expose this as public API, since all the information
contained in it is certainly already public.

Author: jakelishman

qiskit/circuit/__init__.py

@@ -686,6 +686,13 @@
 
     ControlFlowOp
 
+For convenience, there is a :class:`frozenset` instance containing the :attr:`.Instruction.name`
+attributes of each of the control-flow operations.
+
+.. data:: CONTROL_FLOW_OP_NAMES
+
+    Set of the instruction names of Qiskit's known control-flow operations.
+
 These control-flow operations (:class:`IfElseOp`, :class:`WhileLoopOp`,
 :class:`SwitchCaseOp` and :class:`ForLoopOp`) all have specific state that defines the branching
 conditions and strategies, but contain all the different subcircuit blocks that might be entered in
@@ -1257,6 +1264,7 @@ def __array__(self, dtype=None, copy=None):
     CASE_DEFAULT,
     BreakLoopOp,
     ContinueLoopOp,
+    CONTROL_FLOW_OP_NAMES,
 )
 
 from .annotated_operation import AnnotatedOperation, InverseModifier, ControlModifier, PowerModifier

qiskit/circuit/controlflow/__init__.py

@@ -25,3 +25,4 @@
 
 
 CONTROL_FLOW_OP_NAMES = frozenset(("for_loop", "while_loop", "if_else", "switch_case"))
+"""Set of the instruction names of Qiskit's known control-flow operations."""

releasenotes/notes/control-flow-op-names-c66f38f8a0e15ce7.yaml

@@ -0,0 +1,5 @@
+---
+features_circuits:
+  - |
+    A new data attribute, :data:`qiskit.circuit.CONTROL_FLOW_OP_NAMES`, is available to easily find
+    and check whether a given :class:`~.circuit.Instruction` is a control-flow operation by name.