Turn off append-to-itself optimization if optimizations are turned off.

This optimization triggered a bug when doing
  s = append(s, s)
where we write to the leftmost s before reading the rightmost s.

Update #17039

Change-Id: I21996532d20a75db6ec8d49db50cb157a1360b80
Reviewed-on: https://go-review.googlesource.com/81816
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>