2. Allow negative scales in ConvertableToGEP, so that this loop from
   alloc_tree in the Olden health benchmark is correctly raised:

       for (i = 3; i >= 0; i--)
         fval[i] = alloc_tree(level - 1, label*4 + i + 1, new);

   Stepping through the array backwards caused a negative scale.

llvm-svn: 1922