Allow the "a=?1 OR a=?2" to "a IN (?1,?2)" transformation to work on virtual

tables again.  This was formerly restricted because virtual tables could not
optimize IN terms.  (See check-in [fad88e71cf195e].)  But IN terms are now
used by virtual tables (as of check-in [3d65c70343]) so the restriction can
now be removed.

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