[IR] Deleted unused class.
[IR] BEGIN/END block fixes

* Fixed bugs in IR builder recording END blocks -- now, they
  always add themselves to the top-level scope when RecordEndBlockInstr
  runs. They always use the nearest local-variable scope for
  accessing local vars.

* Turned off all optimization passes for:
  * BEGIN/END blocks
  * scopes that have END blocks
  * scopes that have BEGIN blocks
  * scopes whose nearest local variable scope have BEGIN blocks

  This strategy lets us not complicate the optimization passes
  themselves with BEGIN/END block worries. It is okay to penalize
  scopes that use this ugly language feature.

  This eliminates crashers when running passes on such scopes.