test: add regression test for issue 53439

Fixes #53439

Change-Id: I425af0f78153511034e4a4648f32ef8c9378a325
Reviewed-on: https://go-review.googlesource.com/c/go/+/449756
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>

test/fixedbugs/issue53439.go

@@ -0,0 +1,20 @@
+// compile
+//go:build goexperiment.unified
+
+// 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 p
+
+import "unsafe"
+
+func F[T int](v T) uintptr {
+	return unsafe.Offsetof(struct{ f T }{
+		func(T) T { return v }(v),
+	}.f)
+}
+
+func f() {
+	F(1)
+}