Faster AVX2 convolve2D function
Faster AVX2 convolve2D function

Processes data in chunks of 8x2 instead of 16x1 for better speed
on narrow blocks (about 1.5x faster in speed test on blocks
narrower than 16 samples).

Modified loop structure for reduced temporary buffer requirement
(2176 vs 34560 bytes).

Change-Id: Ia64eef50ffcaf6a40483dfe209db5db9731ed5a4

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom