test: add regress test for reported non-monomorphizable example

Fixes #48711.

Change-Id: I2ed1ef5267343d4b9f91da0618905098d178db90
Reviewed-on: https://go-review.googlesource.com/c/go/+/362394
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

test/typeparam/issue48711.go

@@ -0,0 +1,18 @@
+// errorcheck -G=3
+
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+func f[T interface{ ~[]P }, P any](t T) { // ERROR "instantiation cycle"
+	if t == nil {
+		return
+	}
+	f[[]T, T]([]T{t})
+}
+
+func main() {
+	f[[]int](nil)
+}