Change `InstructionEncoding` to use `Size` field to derive the BitWidth
for fixed length instructions as opposed to the number of bits in the
`Inst` field. For some backends, `Inst` has more bits than `Size`, but
`Size` is the true size of the instruction.
Also add validation that `Inst` has at least `Size * 8` bits and any
bits in `Inst` beyond that are either 0 or unset.
Verified no change in generated *GenDisassembler.inc files before/after.
