gh-106581: Fix two bugs in the code generator's copy optimization (#108380)

I was comparing the last preceding poke with the *last* peek,
rather than the *first* peek.

Unfortunately this bug obscured another bug:
When the last preceding poke is UNUSED, the first peek disappears,
leaving the variable unassigned. This is how I fixed it:

- Rename CopyEffect to CopyItem.
- Change CopyItem to contain StackItems instead of StackEffects.
- Update those StackItems when adjusting the manager higher or lower.
- Assert that those StackItems' offsets are equivalent.
- Other clever things.

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Co-authored-by: Irit Katriel <1055913+iritkatriel@users.noreply.github.com>