golang.org/cl/31572 disabled some write barrier optimizations, but
inadvertantly disabled optimizations for some non-pointer composite
literal assignments too.

Fixes #18370.

Change-Id: Ia25019bd3016b6ab58173298c7d16202676bce6b
Reviewed-on: https://go-review.googlesource.com/34564
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>