MergeFunctions Pass, FnSet has been replaced with FnTree.

Patch activates new implementation.
So from now, merging process should take time O(N*log(N)).
Where N size of module (we are free to measure it in
functions or in instructions). Internally FnTree represents
binary tree. So every lookup operation takes O(log(N)) time.

It is still not the last patch in series, we also have to
clean-up pass from old code, and update pass comments.

This patch belongs to patch series that improves MergeFunctions
performance time from O(N*N) to O(N*log(N)).