[SPARK-2366] [SQL] Add column pruning for the right side of LeftSemi join.

The right side of `LeftSemi` join needs columns only used in join condition.
This comment is kind of misleading.  It says that it is collecting the references required above, but actually is only collecting the attributes required for the condition.  Since you are only applying it to the right side, I believe the code is correct, but it would be good to be more clear about the intent.  Also I think off -> all?