Today's earlier fix can stay, but it's a band-aid over the real problem,
which is that bad code was slipping through the type checker
into the back end (and luckily causing a type error there).

I discovered this because my new append does not use the same
temporaries and failed the test as written.

Fixes #9521.

Change-Id: I7e33e2ea15743406e15c6f3fdf73e1edecda69bd
Reviewed-on: https://go-review.googlesource.com/9921Reviewed-by: Ian Lance Taylor <iant@golang.org>