Not for review, simply saving an idea for later.
The idea behind this is the a statepoint operand which can be spilled should contribute to the spill cost of a live interval as it's equally happy to use register and stack.
This made a difference on a larger test I'd looked at, but I haven't managed to reduce a small test case. All of the small test cases I write have obviously bigger problems (quality wise).
Setting this aside until other issues resolved.