Bug fix in probability update savings computation

Found this bug while tracking down some anomalies in my experiments.
Since vp8_cost_one and vp8_cost_zero return unsigned int, the
bit shift by 8 will be incorrect if the value is negative.
I am cautiously optimistic that this fix will make the prob
updates more correct and somewhat improve results across the board.
But the update probabilities will need to be retuned I think.

Patch 2: Adding more of the same fixes using a macro.

Change-Id: I1a168f040e74e8c67e7225103b1c2af9a611da49