RFR: 8290715: Fix incorrect uses of G1CollectedHeap::heap_region_containing() [v2]

[Thomas Schatzl]

> Hi all,
> 
>   please review this change that fixes some callers of `G1CollectedHeap::heap_region_containing` to a) fail on trying to get the `Heapregion*` on uncommitted regions and b) change some callers to use the correct `heap_region_containing_or_null` method that had been intended there.
> 
> Also remove some now unneccessary asserts as `heap_region_containing` will fail when it previously returned `nullptr`.
> 
> Testing: tier1-5
> 
> Thanks,
>   Thomas

Thomas Schatzl has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/9584/files
  - new: https://git.openjdk.org/jdk/pull/9584/files/b6afb444..b6afb444

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9584&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9584&range=00-01

  Stats: 0 lines in 0 files changed: 0 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9584.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9584/head:pull/9584

PR: https://git.openjdk.org/jdk/pull/9584