cmd/compile: simplify memory load and store operations on loong64

cmd/compile: simplify memory load and store operations on loong64

If the memory load and store operations use the same ptr, they are
combined into a direct move operation between registers, like riscv64.

Change-Id: I889e51a5146aee7f15340114bc4abd12eb6b8a1f
Reviewed-on: https://go-review.googlesource.com/c/go/+/700535
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Keith Randall <khr@golang.org>