cmd/compile: fix panic with nested dead hidden closures
CL 342350 fixed deadcode panic with dead hidden closures. However, a
closure may contains nested dead hidden closures, so we need to mark
them dead as well.
Fixes #51839
Change-Id: Ib54581adfc1bdea60e74d733cd30fd8e783da983
Reviewed-on: https://go-review.googlesource.com/c/go/+/394079
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Cherry Mui <cherryyz@google.com>