Improve DCE in liveness.

Improve DCE in liveness.

In particular:
- Rather than only removing dead code as part of dead store removal,
  remove all side-effect-free nodes that either do not produce a value
  or produce a value that is unused.
- When optimizing, set `fgStmtRemoved` in order to indicate that tracked
  lclVar uses or defs have been removed and liveness may need to be
  recalculated. Previously this flag was only set upon eliminating a
  dead store.