The last change was incomplete, in that it did not evaluate the array
    argument in some cases where it had to be evaluated.  This reuses the
    existing code for checking whether len/cap is constant.
    
    Also clean up the use of _ as the second variable in a for/range,
    which was previously inconsistent depending on whether the statement
    used = or :=.
    
    Updates golang/go#22313
    
    Reviewed-on: https://go-review.googlesource.com/91715

From-SVN: r257377