chore: more process_effect tweaks

[Rich-Harris]

we can make things a tiny bit smaller/faster

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[Rich-Harris]

we could do this instead which would presumably be a tiny bit faster, but less readable

Suggested change

	var is_skippable_branch = is_branch && (flags & CLEAN) !== 0;
	var is_skippable_branch = (flags & (CLEAN | BRANCH_EFFECT)) === (CLEAN | BRANCH_EFFECT);