Begin to implement the Simple-V loop

Begin to implement the Simple-V loop

After returning from executing an instruction, decide whether to return
to Fetch, or go repeat Execute again.

1) If PC or SVSTATE were updated, go directly to Fetch, without updating
   either
2) If there is no vector output, or it's the last VL loop iteration, go
   back to Fetch as well, but update the PC. In the latter case, also
   reset SRCSTEP
3) Otherwise, we are still in the loop, so increment SVSTEP, and go back
   to Execute. But, first, pass through a new state, DECODE_SV, so the new
   register numbers can be decoded.