Speed up compound inter prediction mode check

This commit allows the encoder to store outcomes of single reference
frame modes and compares them to decide if the inter prediction
filter, forward transform, and quantization can be skipped.

The compression performance of speed 3 is down
derf  -0.364%
stdhd -0.198%

For test sequences, the speed 3 runtime is reduced
highway CIF 100 kbps, 51976 ms -> 45033 ms, 13% speed-up
stockholm 720p 1000 kbps, 71826 ms -> 67838 ms, 5.5% speed-up
pedestrian 1080p 2000 kbps, 154924 ms -> 150702 ms, 2.6% speed-up

Change-Id: I5aa26f918d2b4b5197a2c0afa2779319f1c88e44