GCSEing nodes into unreachable blocks is ok now. The
verifier does not check the cfg, when irg is floating.
Code placement fixes and pins the irg after gcse.
Nodes in unreachable blocks are moved into the first block
after the start block.

Reachability checks in place_late are converted into asserts,
because after place_early, there are no unreachable blocks anymore.

reverts c64fe299 and 318579c4
but also fixes opt/unreachable_block