cmd/compile: in clobberdead mode, don't clobber slots that are live for defers

There are stack slots that are kept live for defers, which are
tracked separately. Don't clobber them.

Change-Id: Ib558345758b5a4fd89c7ff8a3fe08087059add21
Reviewed-on: https://go-review.googlesource.com/c/go/+/310329


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>