8223794: applications/kitchensink/Kitchensink.java crash bad oop with Graal

Added new nmethod::oop_at_phantom() method for JVMCI to notify GC that oop should be kept alive

Reviewed-by: dlong, eosterlund

src/hotspot/share/code/nmethod.hpp

@@ -392,6 +392,7 @@ class nmethod : public CompiledMethod {
   // Support for oops in scopes and relocs:
   // Note: index 0 is reserved for null.
   oop   oop_at(int index) const;
+  oop   oop_at_phantom(int index) const; // phantom reference
   oop*  oop_addr_at(int index) const {  // for GC
     // relocation indexes are biased by 1 (because 0 is reserved)
     assert(index > 0 && index <= oops_count(), "must be a valid non-zero index");