Make re-assembly more robust

[travitch]

Currently, renovate re-assembles basic blocks that the caller has not changed to produce the bytes that will be written back to the new binary. We could instead just copy the bytes from the original, which would be robust against re-assembly bugs (especially relevant for the x86_64 backend).

We would need to be a bit careful here to ensure that this is only applied to completely unchanged blocks (i.e., that the block successor does not need to change).

It isn't clear that this would be a significant win, but it could avoid some annoying re-assembly problems.