specifying insn costs from attributes

[Tomas Svensson]

In the .md-file of my port, I have set an attribute "size" of every
insn, giving its size (obviously), in bytes. Is there any way I can use the
value of this attribute to determine the cost (in e.g. TARGET_RTX_COSTS)
when optimizing for size? Or is there some other smart way of achieving
the same thing? Is it better to use the "length" attribute for this?