x86: drop stray W

x86: drop stray W

The flag is used to indicate opcodes which can be switched between byte
and word/dword/qword forms (in a "canonical" way). Obviously it's quite
odd then to see it on insns not allowing for byte operands in the first
place. As a result the opcode bytes need to be adjusted accordingly,
which includes comparisons done in optimize_encoding().

To make re-introduction of such issues less likely have i386-gen
diagnose it (in a generally non-fatal way for now).