op_next is now set up by B::Replace.
Fix handling of non-jittable subtrees.

Only keep around non-jittable optrees rather than the whole
jitted optree, and link non-jittable optrees as kids of the
generated op, more or less using the same structure as the previous
incarnation of the jit.

This is likely to expose more bugs in the optree manipulation,
but they need fixing rather than papering over...
More warnings WTF fixes.