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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.

Optimizing arithmetic with Unknown nodes to a new Unknown node is
illegal unless you can prove that the original Unknown node has no
other users.

We previously used NoMem as dependency for the stores of parameters
whose address is taken. This was wrong as obviously other code inside
the inlined function may change the value which might make a dependency
necessary in case of loops.

This fixes firm-testsuite/opt/fehler228.c

This fixes opt/combo8.c.

Now mode_b behaves the same as integer modes, so use the same code to initialize the special values.

The removed special case used the same color anyway.

The new code in bitop_chain did not consider Add nodes that behaved like
logical ops.

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!

The rule that a rule becomes undefined if it has an undefined input is
only true if the node has no side effects.

This fixes opt/combo7.c