Fix CSE in back-end

[hermanventer]

Release note: none

This fixes two problems uncovered while debugging the failure of PR #2460 when run on a large internal test case:

1. CSE did not fix up recently introduced aliases of expressions.
2. The serializer logic visited array elements that were not visited because of recently introduced leak logic.

[sb98052]

Thanks. This fixes a very gnarly problem that has also been blocking me. On the the leaking logic - I see the issue - we can only omit the serialization of properties as literals if we know them to have been materialized. This does not include the index properties of an array. I will investigate this further and follow up.

[facebook-github-bot]

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