The front-end of the bytecode compiler produces a broken CFG.
A number of "basic-block"s have terminators before their end.
This makes the back-end optimizations unsafe as they rely of a well-formed CFG.

The fix is simple. Insert a check that the CFG is well-formed before doing any optimizations, then fix up the front-end.

Once done, we can be more aggressive with optimizations without worrying that things will break for no apparent reason.