MergeFunctions Pass, introduced total ordering among GEP operations.

Patch replaces old isEquivalentGEP 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)).

llvm-svn: 208976