cmd/compile: fix panic with dead hidden closures

Currently, for hidden closures, we always push them to compile queue
during typechecking. If the hidden closure is discarded from the outer
function body during deadcode, any desugaring phase after deadcode won't
be applied to the closure. Thus, some un-expected OPs are passed to
downstream passes, which they can't handle, the compiler goes boom!

To fix this, we keep track of discarded hidden closures during deadcode
pass, and won't compile them then.

Fixes #47712

Change-Id: I078717d5d1f4f2fa39cbaf610cfffbb042e70ceb
Reviewed-on: https://go-review.googlesource.com/c/go/+/342350


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>