A switch statement without a tag requires case values to be bools, but
the parser does not enforce that, so AST-walking code needs to take
care.

Change-Id: I7d9abbb0324314e02a37813c2d2f6adb0d6af5e7
Reviewed-on: https://go-review.googlesource.com/107375Reviewed-by: Rob Pike <r@golang.org>