This commit skips the compound inter mode prediction check in the
rate-distortion optimization loop for ARF coding. It reduces the
runtime for certain test clips at speed 3, at no compression
performance change:

bus CIF 1000 kbps, 8260 ms -> 8090 ms, 1.8% speed-up
stockholm 720p 1000 kbps, 74453 ms -> 71826 ms, 2.9% speed-up

No visible speed-up for pedestrian area 1080p at 2000 kbps.

Change-Id: Ic68aa56837159b726563b784e2e3729e846465ad