chore: tidy up each block compiler logic

[Rich-Harris]

per #12511, I intend to look into whether we can use compile-time analysis to determine the behaviour of each blocks. Just doing a little tidy-up before I get started:

• as far as I can see, node.context being a destructured pattern has no bearing on whether the index should be reactive
• the reactivity or otherwise of the index should likewise have no bearing on whether we can use the fast path optimisation
• removed an unused variable
• renamed each_type to flags, to make it clearer that it's the same bitmask technique used elsewhere (as opposed to e.g. just keyed or indexed, as suggested by 'type')

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[trueadm]

as far as I can see, node.context being a destructured pattern has no bearing on whether the index should be reactive

This worries me a bit. I feel like we've either somehow worked around this issue, or it still exists in some permutation. It would be good for one of us to play around a bit with destructuring and referencing indexes from that in each blocks to see if this might break something.

[Rich-Harris]

Do you have any recollection of what problem it was solving? I can't imagine we'd have added that logic if there wasn't a test that failed without it, and I can't think of any reason it would be necessary

[trueadm]

I think we need some more snapshot tests around each blocks, especially around key blocks that reference things like index. However, we can maybe do that after merging this.