cmd/internal/obj/loong64: reclassify three-register operation instructions and two-register operation instructions

The instructions belonging to case 32 have the same structure as the
instructions in case 2.

The instructions in case 33 are actually two-register operation
instructions. We move their definitions from function oprrr to oprr and
merge their implementation into case 9.

Change-Id: Id04aaa497e78d8198a58f8d406876d16b3f393a7
Reviewed-on: https://go-review.googlesource.com/c/go/+/565616


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