compiler: cast comparison function result to expected bool type

Otherwise cases like
    type mybool bool
    var b mybool = [10]string{} == [10]string{}
get an incorrect type checking error.

Change-Id: Id00b1821ddfa141d28b18fa58fcccad113a116df
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/274446
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
2 files changed