Cycles: Enable QBVH optimization structure for SSE2 CPUs

This commit enables QBVH optimization structure automatically if rendering
with CPU and SSE2 support is detected.

This brings render time of agent shot back to the speed it used to be before
the watertight intersections commit, single koro and sponza scenes are about
7% faster here.

intern/cycles/render/mesh.cpp

@@ -30,6 +30,7 @@
 
 #include "util_cache.h"
 #include "util_foreach.h"
+#include "util_logging.h"
 #include "util_progress.h"
 #include "util_set.h"
 
@@ -980,6 +981,9 @@ void MeshManager::device_update_bvh(Device *device, DeviceScene *dscene, Scene *
 	/* bvh build */
 	progress.set_status("Updating Scene BVH", "Building");
 
+	VLOG(1) << (scene->params.use_qbvh ? "Using QBVH optimization structure"
+	                                   : "Using regular BVH optimization structure");
+
 	BVHParams bparams;
 	bparams.top_level = true;
 	bparams.use_qbvh = scene->params.use_qbvh;