relax `collapse_vars` on `AST_Exit` (#2855)...First introduced in #1862 to stop assignments to migrate beyond `return` or `throw`. Since then `collapse_vars` has been improved to handle various side-effect-related corner cases.
fix & improve `test/ufuzz.js` (#2815)...- use correct `options` when testing `rename`
- mask arbitrarily assigned function IDs to reduce rate of false positives