Portage/Dependency/Normalize.hs: add pop_common into combine_use_guards

It restores ability to simplify
    a?  ( b? ( d ) )
    !a? ( b? ( d ) )
down to
    b? ( d )

which makes lens ebuild generation better, than original .cabal file.

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