[SelectionDAG] Better ISD::ANY_EXTEND/ISD::ANY_EXTEND_VECTOR_INREG ComputeKnownBits support

Add DemandedElts handling to ISD::ANY_EXTEND and add missing ISD::ANY_EXTEND_VECTOR_INREG handling. Despite the lack of test changes this code IS being used - its just that the ANY_EXTEND ops are legalized later on (typically to ZERO_EXTEND equivalents) so we typically manage to combine later on.