RFR 8226690: SIGSEGV in MetadataOnStackClosure::do_metadata

[coleen.phillimore at oracle.com]

Summary: Dont create nmethod if classes have been redefined since 
compilation start.

The bug was caused by a new nmethod created with an old Method in the 
metadata section.  Added verification (which hit on windows) and NSV in 
the other place where the method can be replaced in the nmethod metadata 
section.

There are some jvmci changes (to vmStructs_jvmci.cpp) that might be 
needed also in the graal compiler.

Tested with tier1-6 and failing test 100 times.

open webrev at http://cr.openjdk.java.net/~coleenp/2019/8226690.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8226690

Thanks,
Coleen