Add a best_yrd shortcut in splitmv mode search.

Encoding of first 50 frames of bus (speed 0) @ 1500kbps goes from
1min6.2 to 1min5.9, i.e. 0.5% faster overall.

Change-Id: I59d8a3b2f0a75010fa041d5e2646c8caac5bd683