test: make codegen/memops.go work with both ABIs

Following CL 309335, this fixes memops.go.

Change-Id: Ia2458b5267deee9f906f76c50e70a021ea2fcb5b
Reviewed-on: https://go-review.googlesource.com/c/go/+/309552


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>