compiler: don't insert write barriers if we've seen errors

compiler: don't insert write barriers if we've seen errors

    The compiler skips the escape analysis pass if it has seen any errors.
    The write barrier pass, especially the check-escapes portion, relies
    on escape analysis running.  So don't run this pass if there have been
    any errors, as it may cause further unreliable error reports.

    Reviewed-on: https://go-review.googlesource.com/90575

From-SVN: r257163