[dev.simd] cmd/compile, simd: add VPSADBW

[dev.simd] cmd/compile, simd: add VPSADBW

This new API is given the name SumAbsDiff, a slightly-longer name for
its canonical abbreviation SAD(Sum-Absolute-Differences).

This instruction has some similar semantic's one, but their semantic is much more
specific and complex: MPSADBW, VDBPSADBW. They should have a more
specific name given this fact.

Change-Id: Ied9144440f82919c3c2d45ae4ce5b961ae91a020
Reviewed-on: https://go-review.googlesource.com/c/go/+/697776
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>