gallivm: Use 8 wide AoS sampling on AVX2.

gallivm: Use 8 wide AoS sampling on AVX2.

v2: Make sure that with num_lods > 1 and min_filter != mag_filter we
still enter the splitting path. So this case would still use 4-wide aos
path (as a side note, the 4-wide aos sampling path could actually be
improved quite a bit if we have avx2, by just doing the filtering with
256bit vectors).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>