compiler: handle set-and-use-temp in nilcheck code

compiler: handle set-and-use-temp in nilcheck code

    Change the code in Unary_expression::do_get_backend that introduces
    explicit nil checks for dereference operations to special case
    set-and-use-temporary expressions. For this case it is better to
    generate an explicit reference of the temp in the final conditional
    (avoids introducing tree sharing).

    Reviewed-on: https://go-review.googlesource.com/81915

From-SVN: r255442