Add IDEA data-flow analysis to guess nullability

Add "if return..." folding to "return if"

j2k/testData/fileOrElement/switch/nestedIfFallback.kt

@@ -1,10 +1,9 @@
 fun foo(i: Int, j: Int): String {
     when (i) {
         0 -> {
-            if (j > 0) {
-                return "1"
-            }
-            return "2"
+            return if (j > 0) {
+                "1"
+            } else "2"
         }
         1 -> return "2"
         else -> return "3"