This doesn't matter too much as these were expanded by the LLVM iterator
facade, but eventually this should enable that facade to be a little
less fancy and should also simplify the dispatch to directly use the
three-way comparison.
One case is a bit subtle and didn't have a comment so I added one to
explain a bit what is going on there.
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Co-authored-by: Jon Ross-Perkins <jperkins@google.com>