RFR (T) 8218601: [AOT] Crash in AOTCodeHeap::mark_evol_dependent_methods

[coleen.phillimore at oracle.com]

Summary: null check nmethod in aot dependencies

Tested with mach5 command line in bug, and tier1,2 in progress. I'll run 
further tests if suggested (need instructions).

open webrev at http://cr.openjdk.java.net/~coleenp/2019/8218601.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8218601

please copy me on the reply since I'm not on compiler-dev.

Thanks,
Coleen