Portage/Dependency/Types.hs: DependIfUse now holds both positive and negative branches

It allows slightly richer representation of dependency tree:
    a? ( d1 ) !a? ( d2 )
now takes only one node in dependency tree.
It will allow simpler implementation of complex optimization passes like:
   a? ( b? ( d1 ) ) !a? ( b? ( d1 ) d2 )
 into
   b? ( d1 ) !a? ( d2 )

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