math/big: optimize the implementation of lshVU on loong64

math/big: optimize the implementation of lshVU on loong64

Change-Id: I1aac166aea4f907a7fb93028a39ef9d1e3888c9c
Reviewed-on: https://go-review.googlesource.com/c/go/+/743800
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>