[MachineOutliner][NFC] Traverse suffix tree using a RepeatedSubstring iterator

This takes the traversal methods introduced in r346269 and adapts them
into an iterator. This allows the outliner to iterate over repeated substrings
within the suffix tree directly without having to initially find all of the
substrings and then iterate over them after you've found them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346345 91177308-0d34-0410-b5e6-96231b3b80d8