We lost the m_most_recent_compact_block optimization when removing segwit, because in Core's codebase it used to be gated by `state.fWantsCmpctWitness`. See rABCf7a1e4adc8a9 and D14876.
As a result m_most_recent_compact_block is computed but unused. Make use of it.
This brings our codebase in line with Bitcoin Core's codebase just before [[https://github.com/bitcoin/bitcoin/pull/24062 | core#24062]] (next backport)