RFR: Have adaptive heuristic include only full cycles in average cycle time

[Zhengyu Gu]

On Wed, 4 May 2022 15:55:38 GMT, William Kemper <wkemper at openjdk.org> wrote:

> We have observed cases when a high percentage of short cut cycles (i.e., when evacuation and update reference phases are skipped) may cause the heuristic to believe it can complete _any_ cycle in the time it takes to complete an abbreviated cycle. Triggering the cycle too late may lead to a degenerated cycle. Abbreviated "learning cycles" are similarly ignored. 
> 
> A command line option has been added to disable this change in behavior and restore the previous behavior of including every cycle's time in the average time.

LGTM

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Marked as reviewed by zgu (Committer).

PR: https://git.openjdk.java.net/shenandoah/pull/139