SpecializeSpirv kept its per-id data (folded values, result types, scalar
types, removed ids, blocks, edges, copies, definitions and the live sets) in
std::map and std::set, and repeats fold, prune and copy propagation over the
whole module until nothing changes. Ids are dense below the header's bound,
so the tables become vectors indexed by id. Lookups of operands that are
literals rather than ids stay bounds-checked, iteration keeps ascending id
order, and an id at or beyond the bound now throws.
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