[IR] Fixes so we can run with passes enabled via -Xir.passes=...

* Don't run additional passes when -Xir.passes=... is set on the
  CLI.

* Turns out we needed a new IRFlag that recorded whether a dyn-scope
  was eliminated for a scope (not just whether it is not required).

* Fixed ordering issues so that rubyspecs run with -Xir.passes=LinearizeCFG
  continues to be green.