vect: Move suggested_unroll_factor applying [PR105940]

As PR105940 shown, when rs6000 port tries to assign
m_suggested_unroll_factor by 4 or so, there will be ICE on:

  exact_div (LOOP_VINFO_VECT_FACTOR (loop_vinfo),
             loop_vinfo->suggested_unroll_factor);

In function vect_analyze_loop_2, the current place of
suggested_unroll_factor applying can't guarantee it's
applied for all cases.  As the case shows, vectorizer
could retry with SLP forced off, the vf is reset by
saved_vectorization_factor which isn't applied with
suggested_unroll_factor before.  It means it can end
up with one vf which neglects suggested_unroll_factor.
I think it's off design, we should move the applying
of suggested_unroll_factor after start_over.

	PR tree-optimization/105940

gcc/ChangeLog:

	* tree-vect-loop.cc (vect_analyze_loop_2): Move the place of
	applying suggested_unroll_factor after start_over.

gcc/tree-vect-loop.cc

@@ -2388,15 +2388,15 @@ vect_analyze_loop_2 (loop_vec_info loop_vinfo, bool &fatal,
      set of rgroups.  */
   gcc_assert (LOOP_VINFO_MASKS (loop_vinfo).is_empty ());
 
+  /* This is the point where we can re-start analysis with SLP forced off.  */
+start_over:
+
   /* Apply the suggested unrolling factor, this was determined by the backend
      during finish_cost the first time we ran the analyzis for this
      vector mode.  */
   if (loop_vinfo->suggested_unroll_factor > 1)
     LOOP_VINFO_VECT_FACTOR (loop_vinfo) *= loop_vinfo->suggested_unroll_factor;
 
-  /* This is the point where we can re-start analysis with SLP forced off.  */
-start_over:
-
   /* Now the vectorization factor is final.  */
   poly_uint64 vectorization_factor = LOOP_VINFO_VECT_FACTOR (loop_vinfo);
   gcc_assert (known_ne (vectorization_factor, 0U));