This was fun to track down

[Torbjorn Granlund]

     That change is wrong.  It is not the right way to fix the observed
     problem.  Jeff, I think you should take it out again.

  Could you please tell me what specifically is wrong with the change?

Sure.

1. It seems strange that convert_modes put a constant into a register.
   Maybe that is a bug.
2. If convert_modes is really right, the proper fix to expand_divmod
   would be to avoid calling it when op1 is constant.

Torbjorn