test: add test for package-scope method value GC

The Go 1.18 frontend handles package-scope generic method values by
spilling the receiver value to a global temporary variable, which pins
it into memory. This issue isn't present in unified IR, which uses
OMETHVALUE when the receiver type is statically known.

Updates #54343.

Change-Id: I2c4ffeb125a3cf338f949a93b0baac75fff6cd31
Reviewed-on: https://go-review.googlesource.com/c/go/+/422198
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>

test/fixedbugs/issue54343.go

@@ -0,0 +1,45 @@
+// run
+
+// 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
+
+import "runtime"
+
+func main() {
+	if wait() {
+		panic("GC'd early")
+	}
+	m = nil
+	if !wait() {
+		panic("never GC'd")
+	}
+}
+
+var m = New[int]().M
+
+func New[X any]() *T[X] {
+	p := new(T[X])
+	runtime.SetFinalizer(p, func(*T[X]) { close(done) })
+	return p
+}
+
+type T[X any] int
+
+func (*T[X]) M() {}
+
+var done = make(chan int)
+
+func wait() bool {
+	for i := 0; i < 10; i++ {
+		runtime.GC()
+		select {
+		case <-done:
+			return true
+		default:
+		}
+	}
+	return false
+}