cmd/compile: simplify s390x 'operation with memory operand' rules
cmd/compile: simplify s390x 'operation with memory operand' rules

The generated code remains exactly the same.

Change-Id: Id70af46b6c3c18153789961e5589eab717c3316e
Reviewed-on: https://go-review.googlesource.com/c/go/+/227164
Run-TryBot: Michael Munday <19411bdeea90f848b8e02e9a1c63a7ce7cc699ea@ibm.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

CAFxX/go,akutz/go,CAFxX/go,nawawi/go,nawawi/go,nawawi/go,akutz/go,golang/go,golang/go,cloudflare/go,golang/go,nawawi/go,akutz/go,golang/go,nawawi/go,cloudflare/go,cloudflare/go,cloudflare/go,akutz/go,golang/go,CAFxX/go,akutz/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,cloudflare/go,CAFxX/go,golang/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go,akutz/go,akutz/go,akutz/go,cloudflare/go,nawawi/go,cloudflare/go