Highbd rectangle intrapred V/DC sse2 optimization

Function speedup (i7-6700),  sse2 verse C:
Predictor      V_PRED    DC_PRED
4x8            ~1.5x     ~4.9x
8x4            ~2.5x     ~4.8x
8x16           ~1.9x     ~9.1x
16x8           ~1.9x     ~4.4x
16x32          ~2.1x     ~5.8x
32x16          ~2.0x     ~3.6x

Change-Id: I6deffd0637e57ee5d0bd533502f5705148c4cdd4