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Build on PosInSet and SetSize functions in ax_node.cc to handle
invalid or erroneous assignment. SetSize should not allow decreasing
assignments, while PosInSet should not allow decreasing nor duplicate
assignments among nodes in the same container.

Change-Id: I07b0e9c493439aca9ef50ecfe3c3f5bbf7a7ce7a
Reviewed-on: https://chromium-review.googlesource.com/c/1343374
Commit-Queue: Akihiro Ota <akihiroota@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610920}