cmd/internal/obj/loong64: simplify buildop

cmd/internal/obj/loong64: simplify buildop

The length of array optab is readily available, remove the sentinel and
sentinel scan, like arm64.

Change-Id: Iceeed22587ce2beeedeb5babdde3474e75d70f89
Reviewed-on: https://go-review.googlesource.com/c/go/+/699095
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>