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The remove_empty_blocks function must be used for a pre-walker, otherwise
it might be called on blocks, which were optimized away. Consider infinite
loops and the Keep edges:

  \ ___
   B   |
    \  /
    Empty
    /
   / Keep Edge!

In this example, we must not call remove_empty_blocks on Empty,
after we called it on B, but a post walker would do that.

- renamed clear_link() to clear_link_and_mark_blocks_removable()
- renamed optimize_conds() to optimize_ifs()
- simplified has_*() implementations
- made some walker POST walker
- invalidate out edges
- and add some doxygen docu
- fixed some typos
- adapt to firm indentation style

The get_n in the foreach loop is a macro and not a use of the function.

merge start-block successor with start-block in backend (necessary after equivalent_node_Block is gone)

Since Bads have modes now the check for mode_X initialized the bitinfo,
which results in incorrect reachability information.

Broken with csmith 2.0.0 seed 909842313
in libFirm 1953ce88

convopt: fix potential bug with modulo shifts, improve cost calculation for optimizable conv(shift) combinations

Fixes C/unkept_loop.c

The functionality already existed,
but this is necessary at more than on place.

        Cond
       /    \
  projA      projB   =>   Jmp     Bad
       \    /                \   /
       block                 block

This optimization might produce more empty blocks to remove,
which in turn might enable more Cond=>Jmp opts.
So we need to perform this until nothing changes anymore.

Fixes x86code/cfopt.c
Breaks C/unkept_loop.c, because ifconv cannot handle Bads

This code does nothing, because the previous commit removed the
equivalent_node_Block function.

Fixes opt/localopts2

Because of Phis blocks cannot be equivalent. The cfopt must do all
block merging.

For example, consider the case where block A has one predecessor
block B, which contains a Jmp. Naively, we might exchange(A,B).
However, A might contain a Phi with one operand, then the graph
is broken afterwards, since we move the Phi into B as well.