* 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.

From-SVN: r24333