loong64: fix self-assignment error

Change-Id: Icdc21032f37ebd56f5bbbbe058637efbc7cef1f0
Reviewed-on: https://go-review.googlesource.com/c/arch/+/611135
Reviewed-by: Qiqi Huang <huangqiqi@loongson.cn>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
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>

loong64/loong64asm/plan9x.go

@@ -47,7 +47,7 @@ func GoSyntax(inst Inst, pc uint64, symname func(uint64) (string, uint64)) strin
 			op = "MOVHU"
 			args = append(args[1:2], args[0:1]...)
 		} else {
-			args[0], args[1], args[2], args[3] = args[2], args[1], args[3], args[0]
+			args[0], args[2], args[3] = args[2], args[3], args[0]
 		}
 
 	case BCNEZ, BCEQZ: