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

[Roman Kennke]

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.

OK, makes sense, then. Thank you!

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Marked as reviewed by rkennke (Reviewer).

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