MergeFunctions Pass, introduced total ordering among operations.

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