Restrict subplan nodes even for equal distributions

Many regression tests were failing because the expected plan contains

    Remote Subquery Scan on all (datanode_1)

but we were producing

    Remote Subquery Scan on any (datanode_1,datanode_2)

Both those plans are in fact valid (at least on replicated tables).
The difference is that in the first case the restriction was computed
in adjust_subplan_distribution() while in the second case this did not
happen (and instead will happen at execution time).

The restriction is not applied because adjust_subplan_distribution()
contains this condition

    if (subd && !equal(subd, pathd))
    {
        ... restrict execution nodes ...
    }

In Postgres-XL 9.6 the two distributions happen to be equal in some
cases where where that was not the case in Postgres-XL 9.5. It's not
entirely clear why this happens (it seems to be another consequence
of the upper-planner pathification), but the consequence is that the
restriction code is skipped.

Removing the equal() call from the condition fixes all the regression
failures caused by plans switching between any/all restrictions. In
fact, this fixes all remaining regressions failures in five regression
suites: create_view, subselect, aggregates, rangefuncs, xc_having.

In the future, we will probably pathify adjust_subplan_distribution(),
i.e. we'll probably get rid of it entirely and compute the restriction
either when constructing the path or possibly defer it until execution
time. Before the upper-planner pathification this was not possible.

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