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some were missing anyway and nobody is using the global declarations anyway

The cfopt itself can handle unreachable code,
but it transforms it in a way that makes the verifier unhappy.

- 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.