x86/hevc_res_add: add ff_hevc_transform_add32_8_avx2

~20% faster than AVX.

Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: James Almer <jamrial@gmail.com>
x86inc: free up variable name "n" in global namespace

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>