* genattrtab.c (check_attr_value): Allow negative const_int if
        negative_ok.  Accept integral arithmetic operators.  Accept
        direct references to other attributes.  Accept symbol_ref in
        non-constant attributes.
        (max_attr_value): Add new argument `unknownp'.  Update all callers.
        (or_attr_value): Likewise.
        (simplify_knowing): Don't optimize if max_attr_value unknown.
        (write_length_unit_log): Likewise with or_attr_value.
        (find_and_mark_used_attributes): Don't fallthru case.
        (write_attr_set): Pass thru all non-cond expressions.
        (write_attr_value): Handle symbol_ref, attr, and arithmetic.

From-SVN: r25348