LocalCSE: Fix regression from #6587 by accumulating generativity (#6591)

#6587 was incorrect: It checked generativity early in an incremental manner, but
it did not accumulate that information as we do with hashes. As a result we
could end up optimizing something with a generative child, and sadly we lacked
testing for that case.

This adds incremental generativity computation alongside hashes. It also splits
out this check from isRelevant.

Also add a test for nested effects (as opposed to generativity), but that already
worked before this PR (as we compute effects and invalidation as we go, already).

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