Return table fields:

• link (number): the linked trace (0 for link types: none, return, interpreter)
• nk (number): the lowest IR constant (???)
• nins (number): the next IR instruction (???)
• linktype (string): the link type (none, root, loop, tail-recursion, up-recursion, down-recursion, interpreter, return)
• nexit (number): number of snapshots (for use with jit.util.tracesnap)