style: Avoid some branches in common RuleIterator::next cases.
style: Avoid some branches in common RuleIterator::next cases.

I saw this function appear in the profiles at #17329.

It was under set_device, which isn't a Stylo path, but probably worth there
anyway.

This reduces the reported overhead of RulesIterator::next in perf from ~8% to
0.46%

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