I'm not really qualified to answer this, but I suspect the instruction decoder(s?) would be decoupled from register state as much as possible (unless x86 is even weirder than I thought).

Given this, I suspect wiring in a path all the way from the relevant versions of the relevant registers might be quite expensive. Plus part of the decode logic now needs to block on a register value - so a timing based attack might find these.

More qualified comments welcome...