Respect receiver-dependent visibility when selecting smart-cast type

compiler/testData/diagnostics/tests/scopes/protectedVisibility/unstableSmartCast.kt

@@ -0,0 +1,18 @@
+// !CHECK_TYPE
+open class BaseOuter {
+    protected fun foo() = 1
+    protected fun bar() { }
+}
+
+class Foo(var base: BaseOuter)
+
+fun BaseOuter.foo(): String = ""
+
+class Derived : BaseOuter() {
+    fun test(foo: Foo) {
+        if (foo.base is Derived) {
+            foo.base.foo() checkType { _<String>() } // Resolved to extension
+            <!SMARTCAST_IMPOSSIBLE!>foo.base<!>.bar()
+        }
+    }
+}