The start block is not special block without any "real" instructions
anymore. In particular this means:

- There is no X_initial_exec Proj from the Start node anymore, "normal"
  cfopts are used for jumping out of the start block.
- The backend never had the special convention of the start block not
  containing actual code and used to fixup this discrpancy, no more
  fixup code needed now.
- The part_block() code got a bit more complicated because splitting the
  start block is tricky (previously part_block() simply failed in this
  case but was never used in the Start block because of its special
  convention)