cmd/internal/obj/loong64: optimize instruction implementation

The plan9 instructions ASLLV and -ASLLV are translated into the same assembly
instructions, so -ASLLV can be removed and replaced with ASLLV in the
corresponding position.

ASRLV and -ASRLV have the same reason as the above two instructions.

Change-Id: I4bd79ca7bb070f7a924a0205ef2f19cf2b9ae2c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/565623


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