Revert "cmd/compile: improve register overwrite decision for resultInArg0 ops"

Revert "cmd/compile: improve register overwrite decision for resultInArg0 ops"

This reverts CL 460656
Reason for revert: This breaks s390x.

Change-Id: I8fada14fabc90593b8033ed11188c04963d2da75
Reviewed-on: https://go-review.googlesource.com/c/go/+/463475
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>