- Simplified the code.

- Removed types for slice aliases from composite literals' whitelist, since they
are properly handled by vet.

Fixes #15408
Updates #9171
Updates #11041

Change-Id: Ia1806c9eb3f327c09d2e28da4ffdb233b5a159b0
Reviewed-on: https://go-review.googlesource.com/22318
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>