KT-3491 Wrong (or not obvious) warning

#KT-3491 Fixed

compiler/testData/diagnostics/tests/dataFlowInfoTraversal/Elvis.kt

@@ -4,6 +4,6 @@ fun foo() {
     val x: Int? = null
 
     bar(x ?: 0)
-    if (x != null) bar(x ?: x)
+    if (x != null) bar(<!USELESS_ELVIS!>x<!> ?: x)
     bar(<!TYPE_MISMATCH!>x<!>)
-}
+}

compiler/testData/diagnostics/tests/nullableTypes/uselessElvis.kt

@@ -0,0 +1,17 @@
+// For KT-3491
+
+fun <T: Any?> test1(t: Any?): Any {
+    return t as T ?: ""
+}
+
+fun <T: Any> test2(t: Any?): Any {
+    return <!USELESS_ELVIS!>t as T<!> ?: ""
+}
+
+fun <T: Any?> test3(t: Any?): Any {
+    if (t != null) {
+      return <!USELESS_ELVIS!>t<!> ?: ""
+    }
+
+    return 1
+}