Fix bug in LVA when running on closures directly.

* LVA usually runs on method scopes and in that context, runs on
  all scopes nested in it.

* But, 2689915405c relaxed the constraint of running LVA on method
  scopes only as part of the series of commits to do more aggressive
  opts on blocks.

* This patch fixes a bug in LVA when LVA runs on a closure directly
  after its dynscope has been eliminated.

* We can now reorder DeadCodeElimination and OptimizeDynScopesPass
  in IRScope:optimizeSimpleScopes without crashing.