cmd/compile: remove the resultInArg0 register checks on linux/loong64

cmd/compile: remove the resultInArg0 register checks on linux/loong64

The automatic resultInArg0 register check has been implemented by CL 296010.

Change-Id: Id0e085ded9aa097bd02593c359a750d938a3aaa6
Reviewed-on: https://go-review.googlesource.com/c/go/+/416034
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Wayne Zuo <wdvxdr@golangcn.org>
Run-TryBot: Wayne Zuo <wdvxdr@golangcn.org>
Reviewed-by: Bryan Mills <bcmills@google.com>