Use outerPlanState macro instead of referring to leffttree

[Richard Guo]

In the executor code, we mix use outerPlanState macro and referring to
leffttree. Commit 40f42d2a tried to keep the code consistent by
replacing referring to lefftree with outerPlanState macro, but there are
still some outliers. This patch tries to clean them up.

Thanks
Richard

v1-0001-Use-outerPlanState-macro.patch
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