[ADT] Refactor post order traversal (#191047)

Currently, po_iterator holds the traversal state. This makes copying
and moving po_iterator fairly expensive and the code cannot be optimized
away in several cases (most of it isn't even inlined in a default
build).

Therefore, refactor post-order traversal to hold the state in a wrapper
class with cheap iterators. Additionally, replace po_storage base class
with a CRTP implementation where users can provide their own storage.

Benefits:

- Performance in stage2-O3 improves by 0.19% instructions:u and even
  more substantially in cycles/wall-time.

- Users that use a custom storage/iteration limitation can do so in a
  more clean way by subclassing PostIteratorTraversalBase. See e.g.
  LoopBlocksTraversal.

- For graphs with block numbers, reserving can now be implemented
  reasonably easy (not done yet).

Implications:

- PostOrderTraversal::iterator is no longer a forward iterator. This
  property was never really used, though.

- PostOrderTraversal must be live while iterators are live. For typical
  uses (for (X x : post_order(...))), this is no problem, but could end
  up being problematic if the iterator is wrapped (e.g.
  for (X x : make_filter_range(post_order(...), ...)) -- problematic,
  because make_filter_range doesn't preserve the range but only the two
  iterators, which become invalid as the for loop is entered). This is a
  limitation of the way LLVM implements ranges.