DFA bug fix: assignment of unstable to stable uses predictable nullability (see test)

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

@@ -0,0 +1,18 @@
+class Foo(var x: Int?) {
+    init {
+        if (x != null) {
+            val y = x
+            // Error: x is not stable, Type(y) = Int?
+            <!SMARTCAST_IMPOSSIBLE!>x<!>.hashCode()
+            y<!UNSAFE_CALL!>.<!>hashCode()
+            if (y == x) {
+                // Still error
+                y<!UNSAFE_CALL!>.<!>hashCode()
+            }
+            if (x == null && y != x) {
+                // Still error
+                y<!UNSAFE_CALL!>.<!>hashCode()
+            }
+        }
+    }
+}

compiler/testData/diagnostics/tests/smartCasts/unstableToStable.txt

@@ -0,0 +1,9 @@
+package
+
+public final class Foo {
+    public constructor Foo(/*0*/ x: kotlin.Int?)
+    public final var x: kotlin.Int?
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}