execfreq: Unbreak build.

error.h is panic.h for quite some time now.
arm: Simplify appending to a single-linked list.
The start block is not special anymore

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)
inline: tweak the meaning of "meaningful node" a bit

Several additional nodes such as NoMem, Pin, Phi[M] are not counted as
instructions anymore, which leads to more inlining which in turn seems
to improve SPEC results slightly.
tailrec: correctly handle mode_P closed form add