Fixed broken SPAWNR and SPAWNDR VM instructions

Both of these had lost the 16:16 ==> integer shift in the transition
to variable length instructions.

Also, in order to match the SPAWNR instruction, SPAWNDR has an odd
encoding using nothing but the 16 bit a2 field for register index,
whereas one would normally use the 8 bit a1 field for that - which
I of course failed to remember...