IMPALA-1281: Restrict re-ordering of cross joins.

This patch restricts the leftmost table ref candidates of cross joins to the
very first join (like we already do for outer/semi joins). Join inversion
is still considered for cross joins.

While conceptually possible, it is tricky to reason about allowing the rhs of
arbitrary cross-join table refs as the leftmost candidate during join
re-ordering. We would have to carefully change the joinOps of all table refs
in between, and ensure to not make those changes in place to avoid "polluting"
the table refs for the next round of join re-ordering (considering a new
leftmost table ref). The safer fix is to restrict the considered orders.

Change-Id: I5fdc323e4a9c2dada06d9aec81769057f7076299
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4438
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins