cmd/compile: use 32x32->64 multiplies on loong64

cmd/compile: use 32x32->64 multiplies on loong64

Gets rid of some sign extensions, like arm64.

Change-Id: I9fc37e15a82718bfcf53db8cab0c4e7baaa0a747
Reviewed-on: https://go-review.googlesource.com/c/go/+/721522
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>