AVX2 gather for lowp

Change-Id: I15f83a72645fed0ed8dca9c9aad66c5db5eb247a
Reviewed-on: https://skia-review.googlesource.com/69920
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
3 files changed