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.

The global analysis has quadratic worst-case complexity in the number of
graphs.  At least if alias analysis is disabled anyway, this can be
avoided.

This makes our usage of PhiMs to represent the observable behaviour of
non-terminating loops more obvious.
Having an explicit loop Phi constructor also fixes problems with Phi
getting optimized before the keep edge was added.

The global switch is not necessary as the specific switches can express
the same options.

This fixes opt/fehler262.c.

For details see endless loops section on the homepage.

- 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 also disables the memcombine optimisation if unaligned stores are
not supported by the backend.
We may add some tests in the future to reenable this in cases where we
can prove that the combined store is aligned.

Rework load memory chain walking:
- We keep the pointer as a base + offset representation all the time
- Reuse predict_load code from localopt for more accurate replacement of
  Loads from constant entities
- Support some more cases where load-after-load and load-after-store did
  not use the same mode but where we can easily get the value with some
  arithmetic.

This fixes opt/loadstore6.c.

Some analysis parts create new nodes destroying the CONSISTENT_OUT_EDGES
property even though no real change has been performed.

This fixes opt/loadstore5.c.

- document this correctly
- correctly test with tarval_is_constant() if you expect a constant
  value.

This commit does:
- Remove tarval_reachable+tarval_unreachable, dataflow analysis rather
  use top/bottom instead of other special values.
- Rename tarval_undefined to tarval_unknown as it corresponds to the
  Unknown node (=a value which we don't know)
- Remove predefined tarval_top/tarval_bottom from tv.h and place the
  defines into combo.c
- Change compute_XXX constant folding functions to return tarval_unknown
  instead of tarval_bad if folding failed
- compute_Unknown in combo results in tarval_bottom == tarval_unknown
  this eliminates strange special cases in the Cond/Switch rules and the
  now unnecessary cprop_X list.

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

This fixes opt/loadstore4.c.

We cannot delete the Store if its address aliases with the source of the
CopyB.

This fixes opt/loadstore3.c.