Avoid redundant operations in `fortran_array_walker'

Avoid redundant operations in `fortran_array_walker'

Move inner dimension's element type determination outside the respective
loops in `fortran_array_walker'.  The operation is exactly the same with
each iteration, so there is no point in redoing it for each element and
while a smart compiler might be able to move it outside the loop it is
regardless a bad coding style.  No functional change.