No rule shall hereafter be promulgated under the limitations of § 8.01-4 or otherwise, which would avoid or preclude the judge before whom an accused is arraigned in criminal cases from hearing all aspects of the case on its merits, or to avoid or preclude any judge in any case who has heard any part of the case on its merits, from hearing the case to its conclusion; provided, however, another judge may hear portions of a case where a judge is required to disqualify himself, in cases in which a mistrial is declared, or in cases which have been reversed on appeal, or in the event of sickness, disability or vacation of the judge. The parties to any suit, action, cause or prosecution may waive the provisions of this section. Such waiver shall be entered of record.