[power] Why is `reserved_stack_space` added twice when computing `slot_offset`?

`reserved_stack_space`  is already taken into account by `frame_size`, and then added on explicitly again for `Incoming` in `slot_offset`.

https://github.com/ocaml/ocaml/blob/03c33f500563f3e12355694f1add98e7bd1096ae/asmcomp/power/emit.mlp#L61