MergeFunctions Pass, introduced total ordering among top-level comparison

MergeFunctions Pass, introduced total ordering among top-level comparison
methods.

Patch changes return type of FunctionComparator::compare() and
FunctionComparator::compare(const BasicBlock*, const BasicBlock*)
methods from bool (equal or not) to {-1, 0, 1} (less, equal, great).

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@211437 91177308-0d34-0410-b5e6-96231b3b80d8