RFR 8209624: [JVMCI] Invalidate nmethods instead of directly unloading them when the InstalledCode is dropped

[Tom Rodriguez]

http://cr.openjdk.java.net/~never/8209624/webrev
https://bugs.openjdk.java.net/browse/JDK-8209624

When the lifetime of an InstalledCode instance is used to control the 
lifetime of the corresponding nmethod it's possible for the nmethod 
still have live activations when the nmethod is unloaded.  This is 
because the weak semantics of the installed_code means it isn't visited 
in the nmethod::oops_do phase.  We would have to sometimes maintain a 
second strong reference which was visited in nmethod::oops_do to make 
that work.  Instead simply invalidate the nmethod and let the sweeper 
unload the nmethod once it no longer has activations.  Note that this 
issue doesn't affect normal compiles requested the CompilerBroker since 
the InstalledCode reference is weak in that case.  The fix was tested 
with a Truffle program that originally exposed the issue.  mach5 runs 
are in progress.

tom