Improving performance of rav1d video decoder

Title undersells post; it’s actually 2.3% faster with two good optimizations.

I think that since the 1.5% one is only for aarch64 it's a bit unfair to claim the full number, more like 1/2 if you consider arm/x86 to be the majority of the (future) deployments

I suppose that’s fair, but I’d give credit for a 2.3% improvement in the test environment. For all we know it may be a net loss in other environments due to quirks (probably not, admittedly).