cmd/compile: fix If lowering on loong64

cmd/compile: fix If lowering on loong64

Update #40724

Co-authored-by: Xiaolin Zhao <zhaoxiaolin@loongson.cn>
Change-Id: I44477e32db765e0299d8361bd2b8d2c95564ed28
Reviewed-on: https://go-review.googlesource.com/c/go/+/521788
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>