Simplify Control Flow Optimisations Cmm pass

It turns out that one of the cases in the optimization pass was
a special case of another. I remove that specialization since it
does not have impact on compilation time, and the resulting Cmm
is identical.
Nuke dead code

  * CmmRewriteAddignments module was replaced by CmmSink a long
    time ago. That module is now available at
    https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/Hoopl/Examples
    wiki page.

  * removeDeadAssignments function was not used and it was also
    moved to the above page.

  * I also nuked some commented out debugging code that was not
    used for 1,5 year.