[dev.typeparams] cmd/compile: fix missing condition in usemethod

CL 330670 simplified usemethod, but dropped the previous condition to
ensure the function have 1 or 2 result. This CL restore that condition,
and also add a test for it.

Change-Id: I434e3736785b43ceea0b386d8d9d01ad78a4ccd2
Reviewed-on: https://go-review.googlesource.com/c/go/+/336609


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>