test: add regression test for issue 54911

It was fixed by CL 422196, and have been already worked in unified IR.

Fixes #54911

Change-Id: Ie69044a64b296f6961e667e7661d8c4d1a24d84e
Reviewed-on: https://go-review.googlesource.com/c/go/+/428758
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

test/fixedbugs/issue54911.go

@@ -0,0 +1,21 @@
+// compile
+
+// Copyright 2022 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
+
+type Set[T comparable] map[T]struct{}
+
+func (s Set[T]) Add() Set[T] {
+	return s
+}
+
+func (s Set[T]) Copy() Set[T] {
+	return Set[T].Add(s)
+}
+
+func main() {
+	_ = Set[int]{42: {}}
+}