If gpdb and orca is build with debugging enabled, there is an assert in
orca to check if the upper and lower bound are both closed.
`GPOS_ASSERT_IMP(FSingleton(), fLowerClosed && fUpperClosed);`

The histogram that is stored in pg_statistics might lead to have a
singleton buckets as follows: `10, 20, 20, 30, 40` which will lead to have
buckets in this format: `[0,10), [10, 20), [20,20), [20,30), [30,40]`

This will cause assert to fail since [20,20) is a singleton bucket but
its upper bound is open.

With this fix, the generated buckets will look like below:
`[0,10], [10,20), [20,20], (20,30), [30,40]`
Signed-off-by: NShreedhar Hardikar <shardikar@pivotal.io>