A set of additional tests for complex expressions DFA

compiler/testData/diagnostics/tests/smartCasts/shortIfExprNotNull.kt

@@ -0,0 +1,9 @@
+fun baz(s: String?): String {
+    val t = if (s != null) s
+    else {
+        val u: String? = null
+        if (u == null) return ""
+        u
+    }
+    return <!DEBUG_INFO_SMARTCAST!>t<!>
+}