Update 'this' extension receiver when there's a smart cast

If 'this' (implicit or explicit) was used as an extension receiver,
and the corresponding call required a smart-cast,
this information was effectively lost in "old" resolution & inference,
but is required by "old" JVM BE to generate proper CHECKCASTs.

compiler/testData/codegen/box/smartCasts/kt19058.kt

@@ -0,0 +1,19 @@
+// TARGET_BACKEND: JVM
+// FILE: Test.kt
+open class KFoo {
+    fun foo(): String {
+        if (this is KFooBar) return bar
+        throw AssertionError()
+    }
+}
+
+class KFooBar : KFoo(), JBar
+
+fun box(): String = KFooBar().foo()
+
+// FILE: JBar.java
+public interface JBar {
+    default String getBar() {
+        return "OK";
+    }
+}

compiler/testData/codegen/box/smartCasts/kt19100.kt

@@ -0,0 +1,12 @@
+open class KFoo {
+    fun foo(): String {
+        if (this is KFooQux) return qux
+        throw AssertionError()
+    }
+}
+
+class KFooQux : KFoo()
+
+val KFooQux.qux get() = "OK"
+
+fun box() = KFooQux().foo()