When you edit the operand of an instruction that targets an in-file
address, you're given the opportunity to specify a shortcut that
applies the symbol to the instruction's target address in addition
to or instead of defining a weak symbol reference on the instruction
being edited.
This didn't work right for operands with adjustments, e.g. the store
instructions in self-modifying code. It put the label at the
unadjusted offset, which does nothing useful.
We now correctly back up to the start of the instruction or multi-
byte data area.
