Change `JUMP_TO_TOP` to a more general backwards jump for tier 2.

[markshannon]

Currently we have the _JUMP_TO_TOP uop which jumps to the second uop in the tier 2 trace, as the first is reserved for _START_EXECUTOR. Since this is a bit of an odd special case, we might as well add a more general jump backward. Doing so would have a few advantages:

• It is less confusing
• It would actually simplify the JIT a bit
• It would allow loop invariant code motion for looping traces.