(a) Previous litigation.--For purposes of this subchapter, an issue has been previously litigated if:
(1) (Deleted by amendment).
(2) the highest appellate court in which the petitioner could have had review as a matter of right has ruled on the merits of the issue; or
(3) it has been raised and decided in a proceeding collaterally attacking the conviction or sentence.
(b) Issues waived.--For purposes of this subchapter, an issue is waived if the petitioner could have raised it but failed to do so before trial, at trial, during unitary review, on appeal or in a prior state postconviction proceeding.