Use JoinNode.getFkPkJoinCardinality in reduceCardinalityForScanNode

IMPALA-12018 adds reduceCardinalityForScanNode to lower cardinality estimation when runtime filter is involved. It calls 
JoinNode.computeGenericJoinCardinality(). However, if the originating join node has FK-PK conjunct, it should be possible to obtain lower cardinality estimate by calling JoinNode.getFkPkJoinCardinality() instead.