RFR: 8253286: Use expand_exact() instead of expand_at() for fixed requests in G1

[Aleksey Shipilev]

On Fri, 18 Sep 2020 08:56:33 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> Hi all,
>> 
>> Please review this small cleanup that uses `expand_exact()` rather than `expand_at()` when requesting a specific
>> region. This will ensure that we get exactly the requested region.
>> Testing:
>> tier1-2 alone and 3-5 with other changes.
>
> Makes sense. Lgtm.

Commit message still has "instaed" typo, probably due to PR title here?

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PR: https://git.openjdk.java.net/jdk/pull/235