RFR: JDK-8298138: Shenandoah: HdrSeq asserts "sub-bucket index (512) overflow for value ( 1.00)"

[Y. Srinivas Ramakrishna]

JBS link: https://bugs.openjdk.org/browse/JDK-8298138
- Fixed a boundary condition that was triggering an assert.
- Added a simple-minded gtest for HdrSeq, which allows one to exercise the asserting code in a debug build.
- Tested with: `CONF=slowdebug make run-test TEST="gtest:BasicShenandoahNumberSeqTest"`

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Commit messages:
 - Merge branch 'master' into shen_numberseq
 - A simple-minded test of HdrSeq which also exercises the problematic
 - Fix a boundary condition issue w/HdrSeq

Changes: https://git.openjdk.org/jdk/pull/11524/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11524&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298138
  Stats: 77 lines in 3 files changed: 74 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/11524.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11524/head:pull/11524

PR: https://git.openjdk.org/jdk/pull/11524