cmd/compile: simplify zerorange on amd64

cmd/compile: simplify zerorange on amd64

Get rid of duffzero and large zeroing cases. We only use this code
for small things now.

Change-Id: Idcf330d0ac6433448efa8e32be7eb7f988e10122
Reviewed-on: https://go-review.googlesource.com/c/go/+/678619
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@google.com>