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Since safely iterating over out edges caches the next edges, we cannot
recursively move Proj nodes along with their operands.
If the Proj node is the cached one this causes the out edges to suddenly iterate
over another block.

This fixes opt/fehler279.c and opt/fehler280.c.

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)

- It was only available in the ia32 backend and broken there. As the
  feature is rarely used, I remove it for now instead of going through
  the trouble of fixing it.

The previous approach put the irg link into the block attributes, so
while saving memory an extra indirection was necessary. Some testing
with valgrind massif+callgrind revealed that a direct link increses peak
memory usage by ~1% while also increasing runtime by ~1%. More
importantly the code gets a bit simpler and some strange corner cases can be
removed with the direct link.

This is necessary as the arm backend needs special calling conventions
(only even register numbers) in these cases.

Avoid unnecessary pointer dereferencing.

Avoid unnecessary pointer dereferencing.

- Compound type selection is performed with Member
- Array indexing is performed with Sel
- Instead of the simpleSel constructor functions you can use the normal Member
  constructors now
- The concept of array element entities has been removed, Sel nodes just
  contain a pointer to the array type now.

This can be used to effectively test aliasing in libfirm.

Deduce the mode automatically from the given entity.

This removes the strange entity/type duality and we get rid of union symconst_symbol.