Fix bogus test for hypothetical indexes in get_actual_variable_range().

Fix bogus test for hypothetical indexes in get_actual_variable_range().

That function was supposing that indexoid == 0 for a hypothetical index,
but that is not likely to be true in any non-toy implementation of an index
adviser, since assigning a fake OID is the only way to know at EXPLAIN time
which hypothetical index got selected.  Fix by adding a flag to
IndexOptInfo to mark hypothetical indexes.  Back-patch to 9.0 where
get_actual_variable_range() was added.

Gurjeet Singh