If copying from a slice to itself, skip the write barriers
and actual memory copies.

This happens in practice in code like this snippet from
the trim pass in the compiler, when k ends up being 0:

copy(s.Values[k:], s.Values[:m])

Change-Id: Ie6924acfd56151f874d87f1d7f1f74320b4c4f10
Reviewed-on: https://go-review.googlesource.com/94023
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>