Protect against min_profitable_iters going negative

We had:

      if (vec_outside_cost <= 0)
        min_profitable_iters = 0;
      else
        {
	  min_profitable_iters = ((vec_outside_cost - scalar_outside_cost)
				  * assumed_vf
				  - vec_inside_cost * peel_iters_prologue
				  - vec_inside_cost * peel_iters_epilogue)
				 / ((scalar_single_iter_cost * assumed_vf)
				    - vec_inside_cost);

which can lead to negative min_profitable_iters when the *_outside_costs
are the same and peel_iters_epilogue is nonzero (e.g. if we're peeling
for gaps).

This is tested as part of the patch that adds support for fully-predicated
loops.

2018-01-13  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* tree-vect-loop.c (vect_estimate_min_profitable_iters): Make sure
	min_profitable_iters doesn't go negative.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r256621

gcc/ChangeLog

@@ -2,6 +2,13 @@
 	    Alan Hayward  <alan.hayward@arm.com>
 	    David Sherwood  <david.sherwood@arm.com>
 
+	* tree-vect-loop.c (vect_estimate_min_profitable_iters): Make sure
+	min_profitable_iters doesn't go negative.
+
+2018-01-13  Richard Sandiford  <richard.sandiford@linaro.org>
+	    Alan Hayward  <alan.hayward@arm.com>
+	    David Sherwood  <david.sherwood@arm.com>
+
 	* doc/md.texi (vec_mask_load_lanes@var{m}@var{n}): Document.
 	(vec_mask_store_lanes@var{m}@var{n}): Likewise.
 	* optabs.def (vec_mask_load_lanes_optab): New optab.

gcc/tree-vect-loop.c

@@ -3663,15 +3663,16 @@ vect_estimate_min_profitable_iters (loop_vec_info loop_vinfo,
 
   if ((scalar_single_iter_cost * assumed_vf) > (int) vec_inside_cost)
     {
-      if (vec_outside_cost <= 0)
-        min_profitable_iters = 0;
-      else
-        {
       min_profitable_iters = ((vec_outside_cost - scalar_outside_cost)
 			      * assumed_vf
 			      - vec_inside_cost * peel_iters_prologue
-				  - vec_inside_cost * peel_iters_epilogue)
-				 / ((scalar_single_iter_cost * assumed_vf)
+			      - vec_inside_cost * peel_iters_epilogue);
+
+      if (min_profitable_iters <= 0)
+        min_profitable_iters = 0;
+      else
+	{
+	  min_profitable_iters /= ((scalar_single_iter_cost * assumed_vf)
 				   - vec_inside_cost);
 
 	  if ((scalar_single_iter_cost * assumed_vf * min_profitable_iters)