The "PC advancement" trick was used just after recognizing that a
breakpoint exception was going to be generated. This trick has had two
points:
1. Guarantee that tb->size isn't zero: there are many places where it's
expected to be non-zero. In fact, that is even stated in the comment
for this field.
2. Try to satisfy disassembler's check for instruction length. To this
end, PC advancement was done for estimated instruction length, but
actually, didn't work properly in variable-instruction-length cases.
Substitute this trick with checking for TB size at the end of
translation. If we get an empty TB then just set tb->size to 1 and skip
disassembling. Setting tb->size to 1 is enough to get correct behaviour,
whereas an empty TB doesn't obviously need to be disassembled.