[IR] Fix interpretation of top-level script bodies.

* Moved execution of top-level script bodies from Interpreter.java
  to IRScriptBody.java and also cleaned up some mess there.

  - This eliminates the ugly hack of generating a synthetic
    InterpretedIRMethod object for this.
  - An additional frame should not be pushed for script bodies.
  - BEGIN/END blocks should also execute in the same frame/scope
    context as the body.
  - Explicit call protocol pass now disabled for IRScriptBody.

* This fixes 5 additional spec errors.
  IR execution mode now down to 2 failures + 2 errors.