Add safe_normalize to cycles, avoid checking length first

This won't give any big speedup,
just avoids redundant sqrtf and may be useful in future.

Differential Revision: https://developer.blender.org/D880

intern/cycles/subd/subd_dice.cpp

@@ -117,8 +117,8 @@ void EdgeDice::stitch_triangles(Patch *patch, vector<int>& outer, vector<int>& i
 		}
 		else {
 			/* length of diagonals */
-			float len1 = len(mesh_P[inner[i]] - mesh_P[outer[j+1]]);
-			float len2 = len(mesh_P[outer[j]] - mesh_P[inner[i+1]]);
+			float len1 = len_squared(mesh_P[inner[i]] - mesh_P[outer[j+1]]);
+			float len2 = len_squared(mesh_P[outer[j]] - mesh_P[inner[i+1]]);
 
 			/* use smallest diagonal */
 			if(len1 < len2)