PR rtl-optimization/17791
	* loop-doloop.c (doloop_modify): Take number of iterations as
	argument.
	(doloop_optimize): Extend or shorten the number of iterations
	when changing mode of counter register.
	* loop-iv.c (lowpart_subreg): Export.
	* rtl.h (lowpart_subreg): Declare.

From-SVN: r88668