Qiskit / qiskit-terra / 5594502856

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Remove `_legacy_format_cache` slot from `CircuitInstruction` (#10417)

This was originally added as a mitigation for performance loss in
consumers of `QuantumCircuit.data` that were still using the legacy
interface.  It is now becoming more critical to minimise memory usage in
the `QuantumCircuit` object, so the extra slot is a luxury we can no
longer afford.

This represents a 12.5% reduction in the inherent memory footprint of
`CircuitInstruction`, though in practice the impact will be smaller,
since the `operation` and `qubits` fields will near-universally have
associated weight themselves (the `clbits` field is _usually_ the empty
tuple, with is a natural singleton untracked by the GC in CPython).
Other ongoing work on making more objects singletons will reduce those
weights, however.

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