[Statepoint] Sink actual_args and gc_args to GCStatepointInst [NFC]

These are the two operand sets which are expected to survive more than another week or so.  Instead of bothering to update the deopt and gc-transition operands, we'll just wait until those are removed and delete the code.

For those following along, this is likely to be the last (major) change in this sequence for about a week.  I want to wait until all of this has been merged downstream to ensure I haven't introduced any bugs (and migrate some downstream code to the new interfaces).  Once that's done, we should be able to delete Statepoint/ImmutableStatepoint without too much work.