Refactor to reduce branches

This also DRYs up the two calls to current_user.authorized_projects.

Clarify code comment

app/finders/projects_finder.rb

@@ -79,32 +79,10 @@ class ProjectsFinder < UnionFinder
     elsif min_access_level?
       current_user.authorized_projects(params[:min_access_level])
     else
-      if private_only?
+      if private_only? || impossible_visibility_level?
         current_user.authorized_projects
       else
-        default_visibility_levels = Gitlab::VisibilityLevel.levels_for_user(current_user)
-
-        # This is an optimization - surprisingly PostgreSQL does not optimize
-        # for this.
-        #
-        # If the default visiblity level and desired visiblity level filter
-        # cancels each other out, don't use the SQL clause for visibility level.
-        # In fact, this then becames equivalent to just authorized projects for
-        # the user.
-        #
-        # E.g.
-        # (EXISTS(<authorized_projects>) OR projects.visibility_level IN (10,20))
-        #   AND "projects"."visibility_level" = 0
-        #
-        # is essentially
-        # EXISTS(<authorized_projects>) AND "projects"."visibility_level" = 0
-        #
-        # See https://gitlab.com/gitlab-org/gitlab/issues/37007
-        if params[:visibility_level].present? && !default_visibility_levels.include?(params[:visibility_level])
-          current_user.authorized_projects
-        else
-          Project.public_or_visible_to_user(current_user)
-        end
+        Project.public_or_visible_to_user(current_user)
       end
     end
   end
@@ -118,6 +96,30 @@ class ProjectsFinder < UnionFinder
     end
   end
 
+  # This is an optimization - surprisingly PostgreSQL does not optimize
+  # for this.
+  #
+  # If the default visiblity level and desired visiblity level filter cancels
+  # each other out, don't use the SQL clause for visibility level in
+  # `Project.public_or_visible_to_user`. In fact, this then becames equivalent
+  # to just authorized projects for the user.
+  #
+  # E.g.
+  # (EXISTS(<authorized_projects>) OR projects.visibility_level IN (10,20))
+  #   AND "projects"."visibility_level" = 0
+  #
+  # is essentially
+  # EXISTS(<authorized_projects>) AND "projects"."visibility_level" = 0
+  #
+  # See https://gitlab.com/gitlab-org/gitlab/issues/37007
+  def impossible_visibility_level?
+    return unless params[:visibility_level].present?
+
+    public_visibility_levels = Gitlab::VisibilityLevel.levels_for_user(current_user)
+
+    !public_visibility_levels.include?(params[:visibility_level])
+  end
+
   def owned_projects?
     params[:owned].present?
   end