RFR: JDK-8298138: Shenandoah: HdrSeq asserts "sub-bucket index (512) overflow for value ( 1.00)" [v2]

[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"`

Y. Srinivas Ramakrishna has updated the pull request incrementally with one additional commit since the last revision:

  - Copyright dates etc.
  - include reorder to alphabetic; don't use/include std:: namespace.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11524/files
  - new: https://git.openjdk.org/jdk/pull/11524/files/fb5cd5d0..a714630c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11524&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11524&range=00-01

  Stats: 14 lines in 2 files changed: 2 ins; 2 del; 10 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