[VPlan] Support extends and truncs in getSCEVExprForVPValue. (NFCI) Handle extends and truncates in getSCEVExprForVPValue. This enables computing SCEVs in more cases in the VPlan-based cost-model, but should compute the matching costs in all cases. GitOrigin-RevId: 60e5b86052ca1103499e87ee32c8228c71b6c753