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/elvis/impossible.kt

@@ -0,0 +1,40 @@
+// !LANGUAGE: +BooleanElvisBoundSmartCasts
+// See KT-20752
+
+class Unstable {
+    val first: String? get() = null
+}
+
+class StringList {
+    fun remove(s: String) = s
+}
+
+fun StringList.remove(s: String?) = s ?: ""
+
+fun String.isEmpty() = this == ""
+
+fun foo(list: StringList, arg: Unstable) {
+    list.remove(arg.first)
+    if (arg.first?.isEmpty() ?: false) {
+        // Ideally should have smart cast impossible here
+        list.remove(<!SMARTCAST_IMPOSSIBLE!>arg.first<!>)
+    }
+}
+
+class UnstableBoolean {
+    val first: Boolean? get() = null
+}
+
+class BooleanList {
+    fun remove(b: Boolean) = b
+}
+
+fun BooleanList.remove(b: Boolean?) = b ?: false
+
+fun bar(list: BooleanList, arg: UnstableBoolean) {
+    list.remove(arg.first)
+    if (arg.first ?: false) {
+        // Ideally should have smart cast impossible here
+        list.remove(<!SMARTCAST_IMPOSSIBLE!>arg.first<!>)
+    }
+}