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- Make API private as it should only be called by backends
- Let ia32 backend create special ia32_Set nodes instead of relying
  on muxes which must not be touched anymore
- Does not create ConvB nodes anymore but instead produces the Cmp
  directly. (All backends did this anyway during code-selection so I was
  able to leave this case out code-selection)
- First collect nodes to lower, then lower them. This avoids robustness
  problems when transforming the graph while at the same time walking
  it.

We have to convert the values the same way the program does.

This fixes opt/fehler194.c.

This also fixes 176.gcc and 300.twolf.

There are cases where we can normalize in 2 directions and both have
their merits. Just do both in this case. The frontend should at some
point set IRG_GRAPH_STATE_NORMALISATION2 to trigger the 2nd direction.

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.

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.

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