KT-6118 Redundant type cast can be not redundant?

#KT-6118 Fixed

compiler/testData/diagnostics/tests/dataFlow/assignment/uninitializedValIsCheck.kt

@@ -0,0 +1,15 @@
+fun test(a: Any?, flag: Boolean, x: Any?) {
+    if (a !is String) return
+    <!DEBUG_INFO_SMARTCAST!>a<!>.length()
+
+    val b: Any?
+
+    if (flag) {
+        b = a
+        <!DEBUG_INFO_SMARTCAST!>b<!>.length()
+    }
+    else {
+        b = x
+        b.<!UNRESOLVED_REFERENCE!>length<!>()
+    }
+}