VECT: Change flow of decrement IV

VECT: Change flow of decrement IV

Follow Richi's suggestion, I change current decrement IV flow from:

do {
   remain -= MIN (vf, remain);
} while (remain != 0);

into:

do {
   old_remain = remain;
   len = MIN (vf, remain);
   remain -= vf;
} while (old_remain >= vf);

to enhance SCEV.

Include fixes from kewen.

This patch will need to wait for Kewen's test feedback.

Testing on X86 is on-going

Co-Authored by: Kewen Lin  <linkw@linux.ibm.com>

  PR tree-optimization/109971

gcc/ChangeLog:

	* tree-vect-loop-manip.cc (vect_set_loop_controls_directly): Change decrement IV flow.
	(vect_set_loop_condition_partial_vectors): Ditto.