Let the block walker enter endless loops only at kept blocks, not Phis.

Let the block walker enter endless loops only at kept blocks, not Phis.

- 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.