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

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

This reverts commit 4e182db5fc876564a4f87a0602c58ea0ddc6e37c (CL 695196),
which is itself a revert of
ec9e1176c3209cf92e73e3deb2d8073fab5ea4d6 (CL 678620).

So this CL is exactly the same as CL 678620, but with a regalloc fix
(CL 696035) submitted first.

Change-Id: I743ab32fa3aa6ef3e1b2b6751a2ef4519139057c
Reviewed-on: https://go-review.googlesource.com/c/go/+/696016
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>