IMPALA-860: Place outer and semi joins at a fixed position in the plan.

The bug: It is generally incorrect to re-order joins across outer/semi joins.
For example, an inner join following an outer join may reduce the cardinality,
so placing the inner join before the outer join during join re-ordering
would be incorrect because the outer join is cardinality preserving
(on one or both sides). The same argument holds for semi joins.
The fix: Place outer and semi joins at a fixed position in the plan based
on where they appeared in the original query. Inner joins to the left/right of
outer/semi joins are still re-ordered properly.

Change-Id: Idae837097b9376473d7f8124eef69b51f612b210
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1909
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins