[FIR] Fix clearing info about DF variable after reassignment

compiler/testData/codegen/box/fir/falsePositiveBoundSmartcast.kt

@@ -0,0 +1,21 @@
+abstract class A {
+    abstract fun foo(): String
+}
+
+class B : A() {
+    override fun foo() = "OK"
+}
+
+class C : A() {
+    override fun foo() = "fail"
+}
+
+fun test(c: C, cond: Boolean): String {
+    var x: A = c
+    if (cond) {
+        x = B()
+    }
+    return x.foo()
+}
+
+fun box(): String = test(C(), true)