[MachineOutliner][NFC] Remove Parent field from SuffixTreeNode

This is only used for calculating ConcatLen. This isn't necessary,
since it's easily derived from the traversal setting suffix indices.

Remove that. Rename CurrIdx to CurrNodeLen to better describe what's
going on.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346349 91177308-0d34-0410-b5e6-96231b3b80d8