cmd/compile: add extra test for the non-mono pass

Just add a test for another function that is not monomorphisable, which
comes from the Featherweight Go paper.

Updates #48018

Change-Id: I664e3f48412b02678e32b50204dc4befae90374c
Reviewed-on: https://go-review.googlesource.com/c/go/+/361262


Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>