Old deopt worked by compiling two copies of every BB, one with
speculations and one without, and stitching the two together.
This has a number of issues:
- doubles the amount of code LLVM has to jit
- can't ever get back on the optimized path
- doesn't support 'deopt if branch taken'
- horrifically complex
- doesn't support deopt from within try blocks

We actually ran into that last issue (see test from previous commit).  So
rather than wade in and try to fix old-deopt, just start switching to new-deopt.

(new) deopt works by using the frame introspection features, gathering up all
the locals, and passing them to the interpreter.