Six bottom-up IR rewrites (const-fold, inline-node, subst, flatten-lets,
subst-lookup, scalar-replace) each hand-listed every op's child positions —
~250 lines of identical "recurse children, rebuild" arms that had to be kept in
sync whenever an op was added. Extract one map-ir-children into ir.clj that
knows each op's child layout; each walk keeps only its genuine specials
(const-fold's invoke/if, inline-node's invoke, subst's local/let alpha-rename,
scalar-replace's invoke/let folds) and delegates the rest.
The combinator is total over the op set, so the walks are now total too: a
couple soundly gain coverage they previously skipped (const-fold now folds
inside :try; subst-lookup now recurses :def inits, which fixes a latent dangling
ref where a dropped const-key-map binding was referenced inside a def). These
are sound — all six are result-preserving optimizations — and 3-mode conformance
+ fixpoint confirm identical program behavior.
map-ir-children is shape-preserving for :try (recurses :catch-body/:finally only
when present, never assoc's nil) so it can't turn a struct node into a phm.
Written with cond/get only, matching the passes' tier, so no new load-order dep.
Predicates (body-closed?/pure?/local-escapes?), the type-threading infer, and the
Janet backend emit stay as-is: their conservative :else defaults / [type node]
threading / host language don't fit a node-rebuilding combinator.
Adds ir-passes-test coverage for folding reaching fn/loop/try bodies. Full gate
green (conformance x3, suite >=4695/88, fixpoint stage1==2==3, inline-sra + devirt).
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