Add a ControlChildIterator to parallel ValueChildIterator

[ashleynh]

          Maybe we can add a ControlChildIterator to parallel ValueChildIterator, that iterates on control flow children, and use that here? It would avoid us needing to update this code each time we add or modify a control flow structure (i.e. it would keep that logic in `iterator.h`.

I mean, in the code here, we could have:

for (auto* child : ControlChildIterator(curr)) {
  this->addUniqueSymbol();
  Super::walk(child);
}

However that won't work for block, I guess... so we need some special casing here anyhow. Maybe it's not worth it then.

Another option might be BlockChildIterator or such, as what we look for here is a child that is a new block scope..?

Originally posted by @kripken in https://github.com/WebAssembly/binaryen/pull/5772#discussion_r1247115668