RFR: Use PLABs for old gen allocations, including promotions. [v2]

Mon Apr 26 23:03:03 UTC 2021

On Wed, 21 Apr 2021 19:33:25 GMT, Bernd Mathiske <bmathiske at openjdk.org> wrote:

>> I think this could be an assert. The region should already be `FREE` at this point.
>
> When debugging, I came across regions that were "trash" but not "FREE".

Turns out that was OK, because the code at hand here is actually always followed by code that does the same (in try_allocate_in()). My code insertion here is thus redundant. I will remove it.

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PR: https://git.openjdk.java.net/shenandoah/pull/30