Merge the lower block into the upper block (instead of the other way round), so we do not turn the upper block, which might still be in the DFS stack, into and ID node.

It does not fix the if-conv problem (there can still be odd keeps into if diamonds) and it also breaks removal of unreachable code, which is only kept at PhiMs.

This reverts commit 351a807ef7c6225b11cc3ac88be60ea0c9db9b7f.

- This is sufficient, because every endless loop must have one kept block.
- This also prevents that the if-conversion enters endless loops at strange points.
  This lead to exchanging a block, which is currently being recursed, so when later returning to the exchanged block (which now is an Id-node) in the walker stuff blew up.

This fixes opt/fehler183.c.