RFR: 8278826: Print error if Shenandoah flags are empty (instead of crashing)

[Dmitry Chuyko]

On Tue, 14 Dec 2021 23:11:46 GMT, Dmitry Chuyko <dchuyko at openjdk.org> wrote:

> ShouldNotReachHere() is replaced by vm_exit_during_initialization() in few places where ShenandoahGCMode and ShenandoahGCHeuristics are examined.
> 
> Generic testing: gc/shenandoah jtreg tests.

Thanks. I pushed a commit that flips the condition in all 3 modes though 'return NULL;' is eliminated only in PassiveMode.

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PR: https://git.openjdk.java.net/jdk/pull/6842