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

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

@@ -0,0 +1,22 @@
+class Bar {
+    fun bar() {}
+}
+
+class Foo(var x: Any) {
+    init {
+        if (x is Bar) {
+            val y = x
+            // Error: x is not stable, Type(y) = Any
+            <!SMARTCAST_IMPOSSIBLE!>x<!>.bar()
+            y.<!UNRESOLVED_REFERENCE!>bar<!>()
+            if (y == x) {
+                // Still error
+                y.<!UNRESOLVED_REFERENCE!>bar<!>()
+            }
+            if (x !is Bar && y != x) {
+                // Still error
+                y.<!UNRESOLVED_REFERENCE!>bar<!>()
+            }
+        }
+    }
+}

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

@@ -0,0 +1,17 @@
+package
+
+public final class Bar {
+    public constructor Bar()
+    public final fun bar(): kotlin.Unit
+    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
+}
+
+public final class Foo {
+    public constructor Foo(/*0*/ x: kotlin.Any)
+    public final var x: kotlin.Any
+    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
+}