Speed up h_predictor_8x8

Relocate the function from SSSE3 to SSE2, Unroll loop from 4 to 2,
and reduce mem access to left.
Speed up by >20% in ./test_intra_pred_speed.

Change-Id: Ib9f1846819783b6e05e2a310c930eb844b2b4d2e
4 files changed