Add tests for cases when add and remove custom accessors for val with smartcasted usage

jps-plugin/testData/incremental/pureKotlin/valAddCustomAccessor/usage.kt

@@ -0,0 +1,8 @@
+package test
+
+public fun usage() {
+    val a = A()
+    if (a.x != null) {
+        a.x.dec()
+    }
+}