8157181: Compilers accept modification of final fields outside initializer methods

Track initialized final field updates; disable constant folding if an update is detected. Enforce final field update rules introduced by JVMS-7 (but only for JDK 9).

Reviewed-by: vlivanov, dnsimon, forax, never, kvn, coleenp

hotspot/src/share/vm/oops/method.hpp

@@ -638,6 +638,9 @@ class Method : public Metadata {
   // valid static initializer flags.
   bool is_static_initializer() const;
 
+  // returns true if the method name is <init>
+  bool is_object_initializer() const;
+
   // compiled code support
   // NOTE: code() is inherently racy as deopt can be clearing code
   // simultaneously. Use with caution.