Refactor probability savings search.

- Avoid excessive copying
- Don't bother searching if no update can possibly offer savings
- Simplify the interface
- Remove the confusing av1_cost_upd256 macro

(cherry picked from libvpx/master commit
19e0b406c9601ec79f96cba654347e48650929ed)

Change-Id: Id9d9676a361fd1203b27e930cd29c23b2813ce59