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Use the _r variants of set_cur_block() and [gs]et_value().

- Do not handle Bad. This is done by unreachable code elimination.
- Handle NoMem.

This fixes opt/loadstore5.c.

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.

This rule produces more nodes than before and is therefore very
dangerous. The original argument that localopt rules will revert this in
non-profitable cases is not good enough as in combination with CSE, etc.
not everything will get reverted.

In the future someone could write an analysis that can detect cases
where the rule will have a positive effect and reenable it for these
cases.

This fixes opt/fehler235.c.