cmd/compile: optimize loads from readonly globals into constants on loong64

cmd/compile: optimize loads from readonly globals into constants on loong64

Ref: CL 141118
Update #26498

Change-Id: I9c4ad2bedc4d50bd273bbe9119a898d4fca95e45
Reviewed-on: https://go-review.googlesource.com/c/go/+/700875
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>