[DAG] SimplifyVCastOp - ensure we select the correct value type from an SDValue operand

As reported on Issue #62234 - we weren't correctly using the SDValue operand to get its value type, resulting in a failure when it came from a SDNode with multiple results

We haven't been able to create a suitable upstream regression test, but its been confirmed by inspection by both myself and @topperc

Fixes #62234