cmd/compile: improve logical shift with MASKEQZ on loong64

cmd/compile: improve logical shift with MASKEQZ on loong64

Using MASKEQZ instruction can save one instruction in calculation of
shift operations.

Reference: https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html

Change-Id: Ic5349c6f5ebd7af608c7d75a9b3a862305758275
Reviewed-on: https://go-review.googlesource.com/c/go/+/427396
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Wayne Zuo <wdvxdr@golangcn.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>