We can't drop Prog entries when we want to print disassembly.

Added a test for -S.

Fixes #14515

Change-Id: I44c72f70f7a3919acc01c559d30335d26669e76f
Reviewed-on: https://go-review.googlesource.com/19930Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>