Recognize runtime.newobject and don't Zero or NilCheck it.

Fixes #15914 (?)
Updates #15390.

TBD: add test

Change-Id: Ia3bfa5c2ddbe2c27c92d9f68534a713b5ce95934
Reviewed-on: https://go-review.googlesource.com/27930
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>