This updates gfc_trans_preloop_setup to handle multiple loops.
The preliminary patches 1..6 have made it quite painless:
 - the condition "this is the the outermost dimension" is changed from
   `dim == loop->dimen - 1'  to `dim == loop->dimen - 1 && loop->parent == NULL'
 - to retrieve information about the parent loop:
    * if we are not in the outermost dimension, the parent loop's gfc_loopinfo
      struct is the current one, same for the gfc_ss struct, and the loop index
      is dim + 1.
    * if we are in the outermost dimension, the parent loop's gfc_loopinfo
      struct is the current's parent field, same for the gfc_ss struct, and the
      loop index is 0.
OK?