MergeFunctions Pass, updated header comments.

Added short description for new comparison algorithm, that introduces
total ordering among functions set.

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



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211456 91177308-0d34-0410-b5e6-96231b3b80d8