Convert remaining uses to typecheckslice.

Passes toolstash -cmp.

Change-Id: I6ed0877386fb6c0b036e8ee5a228433343855abd
Reviewed-on: https://go-review.googlesource.com/20905
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>