[MachineOutliner] Cleanup: move findCandidates out of suffix tree

Doing some cleanup in preparation for some functional changes.
This commit moves findCandidates out of the suffix tree and into the
MachineOutliner class. This is much easier to follow, and removes
the burden of candidate choice from the suffix tree.

It also adds a couple FIXMEs and simplifies building outlined function
names.

llvm-svn: 309334