RFR: JDK-8298138: Shenandoah: HdrSeq asserts "sub-bucket index (512) overflow for value ( 1.00)" [v3]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:

 - Merge branch 'master' into shen_numberseq
 - - Copyright dates etc.
   - include reorder to alphabetic; don't use/include std:: namespace.
 - Merge branch 'master' into shen_numberseq
 - A simple-minded test of HdrSeq which also exercises the problematic
   code.
 - Fix a boundary condition issue w/HdrSeq

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/11524/files
  - new: https://git.openjdk.org/jdk/pull/11524/files/a714630c..a0edcbda

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11524&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11524&range=01-02

  Stats: 14012 lines in 340 files changed: 9690 ins; 3152 del; 1170 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