Previously a Bad node was returned which could hide problems and more
importantly you create new nodes (the BadBB) in an operation that apparently
only queries the graph.

It just calls optimize_graph_df().
Its documentation was that of optimize_graph_df() anyway.

You can always revive it from older revision, although I would strongly
suffest to start from scratch.

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

Along the way:
- Simplified code to sharpen Cmp relations
- Do not remove unreachable code by using bit information anymore

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 doesn't change anything, but let's gdb print ir_graph_properties
variables/member in a nicer way.

Note that for OO code you should use liboo which provides the same
features!

This can be used to effectively test aliasing in libfirm.

= is_entconst(), is_typeconst, get_entconst_entity() and
get_typeconst_type()

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