Enable adaptive pred filter type for sub8x8

Enable adaptive pred filter type for sub8x8

This commit enables an adaptive prediction filter type selection
for sub8x8 block sizes. In speed 1, it re-uses the filter type of
collocated 8x8 block if it is tested in the rate-distortion optimization
loop, for the sub8x8 blocks. Otherwise, it runs the normal test
over all the three filter types. In speed 2, it re-uses the 8x8
block's prediction filter type, if available. Otherwise, force it
to be EIGHTTAP.

Compression and speed performance wise:
speed 1
derf -0.266%
yt   -0.138%

bus at 2000 kbps: 33766ms -> 30451ms (10% speed-up)
football at 600 kbps: 48173ms -> 43786ms (9% speed-up)

speed 2
derf -0.026%
yt   +0.134%

bus at 2000 kbps: 18973ms -> 17698ms (6% speed-up)
football at 600 kbps: 26748ms -> 25096ms (6% speed-up)

Change-Id: I77e097533b969fd3472147225fa79fc98095d342