[Open-graphics] Multipliers in oga1hq

[Petter Urkedal]

I think we can afford to use 8 bits for encoding registers.  That leaves
us 8 bits of the instruction word for other uses, which is sufficient in
the current version.  Further, we don't need to split evenly between
register kinds, so maybe something like

    q0..q31   previous frame registers r0..r31
    r0..r63   current frame registers
    g0..g127  global registers
    s0..s31   IO-registers

would be more useful.  We should keep in mind, though, that we no longer
have computed addresses, so we can't e.g. do table lookups.  That is,
unless we have a spare memory block which we connect to two of the
IO-registers.  (We have not quite walked in circles.)