RFR: 8297503: Merge FilteringDCTOC into ContiguousSpaceDCTOC

[Albert Mingkun Yang]

Simple cleanup of trimming unnecessary type hierarchy levels.

Test: hotspot_gc

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

Commit messages:
 - s1-flatten-type-hierarchy

Changes: https://git.openjdk.org/jdk/pull/11322/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11322&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297503
  Stats: 52 lines in 2 files changed: 8 ins; 29 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/11322.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11322/head:pull/11322

PR: https://git.openjdk.org/jdk/pull/11322