// IR forming a call graph with a diamond of triangle-shaped SCCs:
//
// d1
-// / \
+// / \
// d3--d2
-// / \
+// / \
// b1 c1
-// / \ / \
+// / \ / \
// b3--b2 c3--c2
// \ /
// a1
-// / \
+// / \
// a3--a2
//
// All call edges go up between SCCs, and clockwise around the SCC.