cmd/internal/obj/loong64, cmd/asm: remove useless instructions

cmd/internal/obj/loong64, cmd/asm: remove useless instructions

Change-Id: I180c40898672a757d72cd0ef38e6e8cc20dc4c3d
Reviewed-on: https://go-review.googlesource.com/c/go/+/565618
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Qiqi Huang <huangqiqi@loongson.cn>
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
Reviewed-by: Cherry Mui <cherryyz@google.com>