[Explain Plan] test_count_distinct fails

This is a regression from build 5.1.0-1394.

Query issued by the test:
explain select count(distinct VMs[1].os) from default where VMs[1].os='windows' or tasks_points.task1>1

Assert which fails:
self.assertTrue( plan['~children'][0]['scan']'#operator'=='IndexScan2')

Indexes created by the test:
CREATE INDEX idx ON default(job_title) where email like '%@%.%' USING GSI
CREATE INDEX idx2 ON default(VMs[1].os,tasks_points.task1) USING GSI
CREATE INDEX idx3 ON default(VMs[1].os,name) where tasks_points.task1=1 USING GSI

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