freedreno/ir3: Allow high registers in sources

I have no idea if these are allowed in destinations, but they are allowed to be propagated by copy propagation, so codify the current de-facto rules by allowing them on sources. If it turns out that some sources don't handle these, or that some additional sources do allow them on a6xx, then we can rework it later. This fixes "INVALID FLAGS" spam with compute shaders in debug mode.