Portage/Dependency/Normalize.hs: enhance 'combine_use_counterguards'

to handle more complex cases like that

  a? ( x y a )
  !a? ( x y na )

yields:

  x
  y
  a? ( a )
  !a? ( na )

Positive results are seen on packages like

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>