KT-7452 Code completion does not honor smart casts for 'this'

#KT-7452 Fixed

idea/idea-completion/testData/basic/common/SmartCastThis.kt

@@ -0,0 +1,21 @@
+class C {
+    fun bar(){}
+}
+
+open class X {
+    fun Any.foo() {
+        if (this is C && this@X is Y) {
+            <caret>
+        }
+    }
+}
+
+class Y : X() {
+    fun y()
+}
+
+fun C.extFun(){}
+
+// EXIST: bar
+// EXIST: y
+// EXIST: extFun