[IR] Selectively run opt passes on certain scopes in default mode

* On method scopes that don't leak bindings, selectively run
  - dead code elimination
  - convert local vars to temp vars

* Speeds up IR interpretation across the board.
  - ~3% speedup across total run of 8 benchmarks
    (375 -> 365 secs on my laptop)
    This is about 38% slower than -X-C

  - red black tree is now ~12% slower than -X-C -- closest to
    AST interpreter it has ever been.