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This also ensures the invariant that at least 1 block in an endless
loop is kept.

optimize_graph_df: iterate marking of unreachable blocks to guarantee that all unreachable code is killed

This fixes opt/fehler186.c and ack/test2.c.

Memory loops without Phis can occur in unreachable code.
Make ldstopt handle this by ignorance.

jump threading must also copy Keep edges to keep End reachable.

Ensure that the result of a node is undefined if any of its operands is undefined (except for phis) or if its block is unreachble.

Correctly handle the case that unreachble blocks are not necessarily replaced before the nodes they contain.

One more condition is needed to ensure monotony.

In older libFirm version, Cmp computes top, true or bottom.
Now Cmp computes true or false, so the monotony fix must be upgraded.

This fixes opt/combo_paper.c

BugFix: mode_b lowering might create new control flow (if ir_create_cond_set() is used), so invalidate analysis data.