Add print-cfg tests

tests/cfg/unused_node.cton

@@ -0,0 +1,16 @@
+; For testing cfg generation where some block is never reached.
+
+function not_reached(i32) -> i32 {
+
+ebb0(v0: i32):
+    brnz v0, ebb2       ;;;; ebb0:inst0 -> ebb2
+    trap
+
+ebb1:
+    v1 = iconst.i32 1
+    v2 = iadd v0, v1
+    jump ebb0(v2)       ;;;; ebb1:inst4 -> ebb0
+
+ebb2:
+    return v0
+}