Improve convolve AVX2 intrinsic for speed

Improve convolve AVX2 intrinsic for speed

This CL refactors the code related to convolve function.
Furthermore, improved the AVX2 intrinsic to compute
convolve vertical for w = 4 case, and convolve horiz for
w = 16 case.

Please note the module level scaling w.r.t C function
(timer based) for existing (AVX2) and new AVX2 intrinsics:

Block     Scaling
Size   AVX2       AVX2
     (existing)   (New)
4x4    5.34x      5.91x
4x8    7.10x      7.79x
16x8  23.52x     25.63x
16x16 29.47x     30.22x
16x32 33.42x     33.44x

This is a bit exact change.

Change-Id: If130183bc12faab9ca2bcec0ceeaa8d0af05e413