[FIR] Don't save DFA implications for unstable local vars

^KT-57502 Fixed

compiler/testData/diagnostics/tests/smartCasts/variables/localDelegatedProperty.kt

@@ -0,0 +1,13 @@
+// FIR_IDENTICAL
+// WITH_STDLIB
+// ISSUE: KT-57502
+
+import kotlin.properties.Delegates
+
+fun test(foo: Any) {
+    var test by Delegates.observable(true) { property, oldValue, newValue ->  }
+    test = foo is String
+    if (test) {
+        foo.<!UNRESOLVED_REFERENCE!>length<!> // no smartcast
+    }
+}