compiler: omit a couple of write barriers

compiler: omit a couple of write barriers

    Omit a write barrier for
        s = s[0:]
    for a slice s.  In this case the pointer is not changing and no write
    barrier is required.

    Omit a write barrier for
        s = append(s, v)
    in the case where len(s) < cap(s) (and similarly when appending more
    values).  When the slice has enough capacity the pointer is not
    changing and no write barrier is required.

    These changes are required to avoid write barriers in the method
    randomOrder.reset in the runtime package.  That method is called from
    procresize, at a point where we do not want to allocate memory.
    Otherwise that method can use a write barrier, allocate memory, and
    break TestReadMemStats.

    Reviewed-on: https://go-review.googlesource.com/134219

From-SVN: r264259