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

#KT-16133 Fixed

j2k/testData/fileOrElement/switch/nestedIfFallback.kt

@@ -0,0 +1,12 @@
+fun foo(i: Int, j: Int): String {
+    when (i) {
+        0 -> {
+            if (j > 0) {
+                return "1"
+            }
+            return "2"
+        }
+        1 -> return "2"
+        else -> return "3"
+    }
+}