Refactor to reduce branches

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

Clarify code comment

app/finders/projects_finder.rb

@@ -79,18 +79,30 @@ 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)
+        Project.public_or_visible_to_user(current_user)
+      end
+    end
+  end
+
+  # Builds a collection for an anonymous user.
+  def collection_without_user
+    if private_only? || owned_projects? || min_access_level?
+      Project.none
+    else
+      Project.public_to_user
+    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 fact, this then becames equivalent to just authorized projects for
-        # the user.
+  # 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))
@@ -100,22 +112,12 @@ class ProjectsFinder < UnionFinder
   # 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
-      end
-    end
-  end
+  def impossible_visibility_level?
+    return unless params[:visibility_level].present?
 
-  # Builds a collection for an anonymous user.
-  def collection_without_user
-    if private_only? || owned_projects? || min_access_level?
-      Project.none
-    else
-      Project.public_to_user
-    end
+    public_visibility_levels = Gitlab::VisibilityLevel.levels_for_user(current_user)
+
+    !public_visibility_levels.include?(params[:visibility_level])
   end
 
   def owned_projects?