AVX2 implementation of av1_convolve_x_sr, av1_convolve_y_sr and
av1_convolve_2d_sr have been added.

Improvements have been made to av1_convolve_x_avx2, av1_convolve_y_avx2
and av1_convolve_2d_avx2.

Change-Id: I62a699dd9dcf42de94dd72cc2d43affc0dc31404