Don't register safe cast type info for unstable values

Important: to be removed in 1.3
So #KT-20752 Fixed

compiler/testData/diagnostics/tests/smartCasts/castchecks/impossible.kt

@@ -0,0 +1,25 @@
+// !LANGUAGE: +SafeCastCheckBoundSmartCasts
+// See KT-20752
+
+class Unstable {
+    val first: String? get() = null
+}
+
+class StringList {
+    fun remove(s: String) = s
+}
+
+fun StringList.remove(s: String?) = s ?: ""
+
+fun foo(list: StringList, arg: Unstable) {
+    list.remove(arg.first)
+    if (arg.first <!USELESS_CAST!>as? String<!> != null) {
+        // Ideally should have smart cast impossible here
+        list.remove(arg.first)
+    }
+    val s = arg.first <!USELESS_CAST!>as? String<!>
+    if (s != null) {
+        // Ideally should have smart cast impossible here
+        list.remove(arg.first)
+    }
+}

compiler/testData/diagnostics/tests/smartCasts/castchecks/impossible.txt

@@ -0,0 +1,20 @@
+package
+
+public fun foo(/*0*/ list: StringList, /*1*/ arg: Unstable): kotlin.Unit
+public fun StringList.remove(/*0*/ s: kotlin.String?): kotlin.String
+
+public final class StringList {
+    public constructor StringList()
+    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 final fun remove(/*0*/ s: kotlin.String): kotlin.String
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class Unstable {
+    public constructor Unstable()
+    public final val first: kotlin.String?
+    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
+}