cmd/compile: use zero register instead of specialized *zero instructions on loong64

cmd/compile: use zero register instead of specialized *zero instructions on loong64

Refer to CL 633075, loong64 has a zero(R0) register that can be used to do this.

Change-Id: I846c6bdfcfd6dbfa18338afc13e34e350580ead4
Reviewed-on: https://go-review.googlesource.com/c/go/+/693876
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>