RFR: Allow transition from any old generation state to idle

[Kelvin Nilsen]

On Mon, 3 Apr 2023 18:34:39 GMT, William Kemper <wkemper at openjdk.org> wrote:

> A cancelled GC can return the old generation to `IDLE` from any state. Also, improve assert message.

Thanks.

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Marked as reviewed by kdnilsen (Committer).

PR Review: https://git.openjdk.org/shenandoah/pull/240#pullrequestreview-1369717823