Simplify the way leap handles conditionals.

This change removes the If(...) instruction, and instead introduces a
`condition` attribute for each instruction. There are no user (method
writer) visible changes; this change purely affects the backend (code
generator and interpreter).

By using a condition attribute rather than conditional edges, we can
simplify reasoning about DAGs and hence the code generation process.
The new code generator is much smaller and clearer than the
previous one, while able to generate structured code of the same
quality (mostly, with some optimizations left to implement).