cmd/compile: add a debug flag to enable/disable open-coded defers

For debugging.

Change-Id: I831947376569cd2285b713ad304329951adf60ab
Reviewed-on: https://go-review.googlesource.com/c/go/+/307230


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