When we're stopped at a breakpoint and we want to
continue in reverse, we're not actually going to
execute the instruction at the breakpoint -- we're
going to de-execute the previous instruction.

Therefore there's no need to singlestep before
inserting breakpoints.  In fact it would be a bad
idea to do so, because if there is a breakpoint at
the previous instruction, we WANT to hit it.

Note that this patch is to be applied to the reverse branch.