Simplify the logic in ShouldPrefetchJoinQual()
Simplify the logic in ShouldPrefetchJoinQual()

Now that we have a slice table with the slice parent-child relationships
readily available, we can use it to answer the question "is this slice the
sender of a Motion?". No need to dig into the plan tree looking for
Motions anymore.

Reviewed-by: Soumyadeep Chakraborty <ec19037653195c8fe6913bfa51ee2dd46d490579@pivotal.io>

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