test/fixedbugs: add regress test for inlining failure

Still investigating, but adding the minimized reproducer as a regress
test case for now.

Updates #66261.

Change-Id: I20715b731f8c5b95616513d4a13e3ae083709031
Reviewed-on: https://go-review.googlesource.com/c/go/+/571815
Reviewed-by: Than McIntosh <thanm@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

test/fixedbugs/issue66261.go

@@ -0,0 +1,26 @@
+// run
+
+// Copyright 2024 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 main() {
+	env := func() func(*bool) func() int {
+		return func() func(*bool) func() int {
+			return func(ptr *bool) func() int {
+				return func() int {
+					*ptr = true
+					return 0
+				}
+			}
+		}()
+	}()
+
+	var ok bool
+	func(int) {}(env(&ok)())
+	if !ok {
+		panic("FAIL")
+	}
+}