[MachineOutliner][NFC] Add findRepeatedSubstrings to SuffixTree, kill LeafVector

Instead of iterating over the leaves to find repeated substrings, and walking
collecting leaf children when we don't necessarily need them, let's just
calculate what we need and iterate over that.

By doing this, we don't have to save every leaf. It's easier to read the code
too and understand what's going on.

The goal here, at the end of the day, is to set up to allow us to do something
like

for (RepeatedSubstring &RS : ST) {
 ... do stuff with RS ...
}

Which would let us perform the cost model stuff and the repeated substring
query at the same time.

llvm-svn: 346269