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/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()