RFR: 8253286: Use expand_exact() instead of expand_at() for fixed requests in G1 [v2]

[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 incrementally with one additional commit since the last revision:

  Update assert and add comment

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/235/files
  - new: https://git.openjdk.java.net/jdk/pull/235/files/a19ef0d5..7af7cd5d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=235&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=235&range=00-01

  Stats: 4 lines in 1 file changed: 1 ins; 1 del; 2 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