RFR: 8290715: Fix incorrect uses of G1CollectedHeap::heap_region_containing()

[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

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Depends on: https://git.openjdk.org/jdk/pull/9572

Commit messages:
 - Some more removal of unnecessary checks
 - Initial changes
 - Fix the places where the wrong heap_region_containing() method has been used

Changes: https://git.openjdk.org/jdk/pull/9584/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9584&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290715
  Stats: 22 lines in 5 files changed: 0 ins; 8 del; 14 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