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In `codegen_assert_terminator` we decide if a BB's successor is a candidate for merging, which requires that it be the only successor, and that it only have one predecessor. That result then gets passed down, and if it reaches `funclet_br` with the appropriate BB characteristics, then no `br` instruction is issued, a `MergingSucc::True` result is passed back, and the merging proceeds in `codegen_block`. The commit also adds `CachedLlbb`, a new type to help keep track of each BB that has been merged into its predecessor.
Please read the rustc-dev-guide chapter on Backend Agnostic Codegen.