Use a stable value for function offsets

[bhackett1024]

Currently the offsets we use within functions is the index into the global array of instrumentation sites. This needs to be consistent in order for breakpoints in replay to work, which requires script compilation order to be consistent among recording and different replays. This isn't the case, and isn't something we can reasonably expect to enforce without running into problems --- compilation can be triggered by all sorts of non-deterministic VM activity. We should use bytecode offsets for function offsets instead, like we do in gecko.