More precise diagnostics of smart cast impossible #KT-7240 Fixed

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

@@ -0,0 +1,13 @@
+class Immutable(val x: String?) {
+    fun foo(): String {
+        if (x != null) return <!DEBUG_INFO_SMARTCAST!>x<!>
+        return ""
+    }
+}
+
+class Mutable(var y: String?) {
+    fun foo(): String {
+        if (y != null) return <!SMARTCAST_IMPOSSIBLE!>y<!>
+        return ""
+    }
+}