[dev.simd] simd, cmd/compile: SumAbsDiff -> SumOf8AbsDiff

Change-Id: Ie3359e2ba575a7f220265b6ae921ade3c271c14a
Reviewed-on: https://go-review.googlesource.com/c/go/+/783021
Auto-Submit: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>