Handle if-conversion within endless loops properly by stopping the recursion when reaching the block with the conditional jump.

Fixes C/unkept_loop.c and opt/fehler159.c

Needed by edgjfe.

Make an if instead of a #if 0 on "broken" code and add an comment
that explains why the non-wroken code is not used now.

Allow Bad predecessors of the end block. These are typically artifacts
of non-optimized control flow.

(they are not represented in the firm schedule)