Prepares for lazy AVX code detection by always setting the VEX_2B or VEX_3B prefix in the fast decoder, so that it is available to test in the main interpreter loop.
For the same reason, moves PREFIX_VEX_2B and PREFIX_VEX_3B definitions from decode_private.h to decode.h.
Fixes some latent minor immediate bugs removing a superfluous leading 0.
Issue: #3935