x86: replace Reg8, Reg16, Reg32, and Reg64

x86: replace Reg8, Reg16, Reg32, and Reg64

Use a combination of a single new Reg bit and Byte, Word, Dword, or
Qword instead.

Besides shrinking the number of operand type bits this has the benefit
of making register handling more similar to accumulator handling (a
generic flag is being accompanied by a "size qualifier"). It requires,
however, to split a few insn templates, as it is no longer correct to
have combinations like Reg32|Reg64|Byte. This slight growth in size will
hopefully be outweighed by this change paving the road for folding a
presumably much larger number of templates later on.