Always set in-set for a CFG node in elide-sync pass

- Needs to be non-null even for dummy nodes.

Reviewed by: Andy Zhang
Make the late-init optimization a ModulePass

- Required for correctness, since it inserts new functions

- Was causing a subtle bug when interacting with the elide-cilk-sync
  pass, whereby the __cilk_conditional_sync function would have its
  internal sync "optimized" away.