Half pixel variance further optimized for ARMv6

Half pixel interpolations optimized in variance calculations. Separate
function calls to vp8_filter_block2d_bil_x_pass_armv6 are avoided.On
average, performance improvement is 6-7% for VGA@30fps sequences.

Change-Id: Idb5f118a9d51548e824719d2cfe5be0fa6996628
6 files changed