* loop.h (loop_info): New field 'vtop'.
* loop.c (check_dbra_loop): Use loop_info->vtop rather than
scanning loop for vtop.
* unroll.c (subtract_reg_term, find_common_reg_term): New functions.
(loop_iterations): Use them to determine if loop has a constant
number of iterations. Set loop_info->vtop. Don't subtract
common reg term from initial_value and final_value if have a
do-while loop.