RFR(S): 8166836: Elimination of clone's ArrayCopyNode may make compilation fail silently

[Roland Westrelin]

Thanks for taking a look at this.

> I thought we do that. There are several places in loopnode.cpp where we 
> hit assert if graph is bad.

In my case, the graph becomes unschedulable because of anti dependencies
so only after loop opts. Should I open another bug to check
C.failure_reason() in C2Compiler::compile_method() and abort if the
schedule failed?

Roland.