[13] RFR(S) 8223794: applications/kitchensink/Kitchensink.java crash bad oop with Graal

[Vladimir Kozlov]

http://cr.openjdk.java.net/~kvn/8223794/webrev.03/
https://bugs.openjdk.java.net/browse/JDK-8223794

Big thanks to Erik O. for pointing the possible cause of this problem and fix suggestion:

"The JVMCINMethodData::get_nmethod_mirror() function in JVMCI is used to read the nmethod mirror from the nmethod oop 
section. However, the nmethod oops are weak, and not kept alive by GC. Yet strong references are created to the returned 
oop in places, without going through a keep-alive barrier. This could result in creating edges to dead oops, resulting 
in a crash eventually."

I added nmethod::oop_at_phantom() method to notify GC that oop should be kept alive. It is used in JVMCI where it needs 
strong reference. I originally named new method and argument 'strong' but Erik pointed that:

"Maybe s/strong/phantom/g for the new method name and parameters. Otherwise it is the other way around, i.e. when strong 
is true in the current webrev01, we actually read it as a phantom oop ref, and conversely when strong is false, we do 
read it as a strong oop ref. That is a bit confusing."

Running Kitchensink without fix triggers the failure intermittently. With the fix I did not see the failure.

Thanks,
Vladimir