Split control flow codegen in two phases:

Split control flow codegen in two phases:

- First, re2c builds a complex structure where it stores
  all control flow codegen decisions: nested ifs or switches,
  bitmaps or computed gotos, etc.
- Second, this structure is traversed and code is generated.

This differentiation is necessary to compute some statistics
(e.g. used labels) in advance, before code generation.