MergeFunctions Pass, introduced total ordering among values.

This is a third patch of patch series that improves MergeFunctions
performance time from O(N*N) to O(N*log(N)).

This patch description:
Being comparing functions we need to compare values we meet at left and
right sides.
Its easy to sort things out for external values. It just should be
the same value at left and right.
But for local values (those were introduced inside function body)
we have to ensure they were introduced at exactly the same place,
and plays the same role.

In short, patch introduces values serial numbering and comparison routine.
The last one compares two values by their serial numbers.

llvm-svn: 208189