CFGDiff: Simplify/common the begin/end implementations to use a common range helper

(would be nice to revisit the CFG traits and change them to use ranges
rather than begin/end - if anyone wants to do that refactor)

Also use more auto because writing the names of range utilty iterators
isn't helping readability here - they're sort of implementation details
for the most part, especially once you nest a few different filtering
and adapting iterators.