Take control flow into account for checking fall-through switch branches

#KT-16133 Fixed

j2k/testData/fileOrElement/switch/nestedSwitchFallback.kt

@@ -0,0 +1,13 @@
+fun foo(i: Int, j: Int): String {
+    when (i) {
+        0 -> {
+            when (j) {
+                1 -> return "0, 1"
+                2 -> return "0, 2"
+            }
+            return "1, x"
+        }
+        1 -> return "1, x"
+        else -> return "x, x"
+    }
+}