RFR: 8253286: Use expand_exact() instead of expand_at() for fixed requests in G1 [v3]

[Stefan Johansson]

> 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.

Stefan Johansson has updated the pull request with a new target base due to a merge or a rebase. The pull request now
contains three commits:

 - Merge branch 'master' into 8253286-use-expand-exact
 - Update assert and add comment
 - 8253286: Use expand_exact() instaed of expand_at() for fixed requests in G1

-------------

Changes: https://git.openjdk.java.net/jdk/pull/235/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=235&range=02
  Stats: 8 lines in 1 file changed: 1 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/235.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/235/head:pull/235

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