fix: only track block effects when deferred to prevent infinite HMR infinite loops

[hmnd]

fixes #17369

Caused by #17332. Block effects were added to dirty_effects unconditionally even if they run immediately. Then on HMR, revive kept rescheduling them causing an infinite, memory eating loop.

Unsure how best to test this one though...

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Preview: https://svelte-dev-git-preview-svelte-17374-svelte.vercel.app/

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Playground

pnpm add https://pkg.pr.new/svelte@17374

[Rich-Harris]

Thank you! I actually don't think this is quite the right fix — this.is_deferred() is something that changes during traversal, so shouldn't also be read in that phase or it could start to have unpredictable effects. Rather than changing the behaviour of all blocks, I think the trick here is just to make HMR blocks behave like others, which is to say it should be a no-op when the underlying component function hasn't changed.

I've opened #17380 as an alternative

[hmnd]

Ahh didn't think of that! I'll close this one then :)