RFR: 8340524: Remove NarrowPtrStruct

[Kim Barrett]

Please review this change which removes the class NarrowPtrStruct. The only
place it was still being used was as the type of CompressedOops::_narrow_oops.
Instead, we move the members from NarrowPtrStruct directly into
CompressedOops, flattening its structure.

Testing: mach5 tier1-3
Tiers 2&3 run serviceability tests that hit the changes to that component.

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Commit messages:
 - remove NarrowPtrStruct

Changes: https://git.openjdk.org/jdk/pull/21115/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21115&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340524
  Stats: 35 lines in 4 files changed: 4 ins; 7 del; 24 mod
  Patch: https://git.openjdk.org/jdk/pull/21115.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21115/head:pull/21115

PR: https://git.openjdk.org/jdk/pull/21115