dom_fn.nx

dom_fn.nx -- dominator tree on *Function (opt.nx consumer). Separate from dom.nx (which operates on a local Block type declared in that file). dom_fn computes immediate dominators directly on the runtime's shared BasicBlock struct so opt.nx passes (LICM next) can consume it without type translation. Algorithm: Cooper-Harvey-Kennedy "A Simple, Fast Dominance Algorithm" (2001). O(N * E * alpha) in practice, trivial implementation compared to Lengauer-Tarjan. Invariants: DF1 Entry block is its own idom by convention. DF2 Unreachable blocks have idom == NULL. Callers should run opt_sweep_unreachable_function first if they want every block reachable. DF3 idom[i] is indexed by ARRAY POSITION (block_index), not by block.id. Caller uses block_index() to look up an idom from a BasicBlock pointer. DF4 RPO (reverse-postorder) guarantees every dominator is seen before its dominatees during the fixpoint pass; this is the invariant that makes Cooper-Harvey-Kennedy's iterate-to-fixpoint converge in O(d) where d is max dominator-tree depth.