Revert "cmd/compile: use generated loops instead of DUFFCOPY on amd64"

Revert "cmd/compile: use generated loops instead of DUFFCOPY on amd64"

This reverts commit ec9e1176c3209cf92e73e3deb2d8073fab5ea4d6 (CL 678620).

Reason for revert: causing regalloc to get into an infinite loop

Change-Id: Ie53c58c6126804af6d6883ea4acdcfb632a172bd
Reviewed-on: https://go-review.googlesource.com/c/go/+/695196
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>