module_ m rewrites each function body, folding the loop shape that Ast_utils.lower_while produces — a void loop whose body is a single back-edged if (leading test) — into a high-level while loop. The (possibly synthetic) loop label is kept only when the body still branches to it once the back-edge is removed (a "continue"), so a label-less source loop round-trips label-less. A trailing-test loop (ending in a back-edged br_if) has no leading-while equivalent and is left a bare loop.

Folding is the exact inverse of the lowering, so re-lowering reproduces the original loop byte-for-byte and the rewrite always preserves runtime semantics. Meant to run on From_wasm.module_ output, after Recover_dispatch.module_ and before Sink_let.module_ (which would otherwise sink locals into the loop body and hide the shape).