Add missing XLATB group/import

[PromyLOPh]

Have you thought about dropping x86_64.json (i.e. dump everything into a single file) and using import *? It took me a while to understand why newly added instructions (opcodes) did not generate classes until I closely looked at the codegen.

[Maratyszcza]

Additional check are required to avoid generating 32-bit form of XLATB (which is technically valid, but practically useless)

[PromyLOPh]

As far as I understand Intel’s docs addressSize defaults to 64 in long mode and REX.W does not change that (cf section 3.6.1 Operand Size and Address Size in 64-Bit Mode). The address-size prefix 67h does set addressSize=32, but opcodes lists only REX.W. Am I missing something?