Optimize and cleanup obmc predictor and rd search.

Use vpx_blend_a64_hmask and vpx_blend_a64_vmask to speed up
computing the obmc predictor. Clean up calc_target_weighted_pred.

Encoder speedup: 1.3%
Decoder speedup: 6.5%

Change-Id: I0c774fe53d22399e92a10d1daf3af0010d88d2c5
6 files changed