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

During global CSE we now ignore nodes in unreachable blocks,
otherwise cse could pick the wrong representative node
and use unreachable nodes in reachable code.

add_block_cf_input breaks, if the block has two Bad inputs

Skipping Loads might lead to dead code, which is still reference by
out edges.

only set block/phi inputs to bad to kill unreachable code, all other
code will be dead and removed anyway

when calling exchange, set_irn_n, or set_irn_in.

fixes backend/inconsistent_outs test case

They were commented out in commit c4686a6b

Also bugfix: The second case used uninitialized memory for the Bad's mode.

Some Bads were not maintained in cfopt

This reverts commit bcaf3e7900158f4fd3c64dda9ff2d11d652ec474,
because the removed assert now holds again.

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