MergeFunctions Pass, introduced total ordering among operations.

Patch replaces old isEquivalentOperation implementation, and changes type of
comparison result from bool (equal or not) to {-1, 0, 1} (less, equal, greater).

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@208973 91177308-0d34-0410-b5e6-96231b3b80d8