compiler: rewrite Type::are_identical to use flags

compiler: rewrite Type::are_identical to use flags

    A single flags parameter replaces the Cmp_tags and errors_are_identical
    parameters. The existing behavior is unchanged.

    This is a simplification step for future work that will add a new flag.

    Reviewed-on: https://go-review.googlesource.com/c/143019

From-SVN: r265293