It's not super fast to clone a function, but it is faster than
re-parsing the test case file it came from.
Some tests want to mutate the function, and there may be other tests in
the same script that need the original function.
- Add a ir::jumptable module with a JumpTableData struct representing the vector
of destinations.
- Add an entity map of jump tables to the Function.
- Parse and write jump tables in the function preamble.
- Rewrite EBB references in jumptables after parsing.