Add OpSize16 bit, for instructions which need 0x66 prefix in 16-bit mode
The 0x66 prefix toggles between 16-bit and 32-bit addressing mode. So in 32-bit mode it is used to switch to 16-bit addressing mode for the following instruction, while in 16-bit mode it's the other way round — it's used to switch to 32-bit mode instead. Thus, emit the 0x66 prefix byte for OpSize only in 32-bit (and 64-bit) mode, and introduce a new OpSize16 bit which is used in 16-bit mode instead. This is just the basic infrastructure for that change; a subsequent patch will add the new OpSize16 bit to the 32-bit instructions that need it. Patch from David Woodhouse. llvm-svn: 198586
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