cmd/internal/obj/loong64: optimize duplicate optab entries

cmd/internal/obj/loong64: optimize duplicate optab entries

Change-Id: I28b79d178a2ed3d304f0e61613439813c4dcf79e
Reviewed-on: https://go-review.googlesource.com/c/go/+/721600
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>