generate checkcast instruction for 'as' calls; use accessors for accessing fields of other instances of the same class

idea/testData/codegen/checkCast.jet

@@ -0,0 +1,15 @@
+class C(val x: Int) {
+  fun equals(rhs: Any?): Boolean {
+    if (rhs is C) {
+      val rhsC = rhs as C
+      return rhsC.x == x
+    }
+    return false
+  }
+}
+
+fun box(): String {
+  val c1 = C(10)
+  val c2 = C(10)
+  return if (c1 == c2) "OK" else "fail"
+}