cmd/internal/obj/loong64: merge two branch classes into one

When the kind of the operand is TYPE_BRANCH, we cannot determine
whether it is a long branch or a short branch, so we merge these
two classes into one.

Change-Id: I7d7fa8f62ff02791ec3de4e3e3f7610bc9cb1743
Reviewed-on: https://go-review.googlesource.com/c/go/+/565626


LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
Reviewed-by: Qiqi Huang <huangqiqi@loongson.cn>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Michael Knyszek <mknyszek@google.com>