compiler: separate incomparable types from comparable ones

    
    Otherwise we can accidentally and incorrectly mark an actual user type
    as incomparable.  This fixes the gccgo version of
    https://golang.org/issue/17752.  The test case for gccgo is
    https://golang.org/cl/33249.
    
    Reviewed-on: https://go-review.googlesource.com/33292

From-SVN: r242494