RFR: JDK-8234502 : Merge GenCollectedHeap and SerialHeap [v8]

[Lei Zaakjyu]

> JDK-8234502 : Merge GenCollectedHeap and SerialHeap

Lei Zaakjyu 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 12 additional commits since the last revision:

 - Merge branch 'openjdk:master' into serialgc
 - replace a necessary include statement
 - clean up
 - add line-breaks
 - fix include statements
 - add some headers
 - Completely removed 'GenCollectedHeap'
 - Fix 'young_gen' function in 'genCollectedHeap.cpp'
 - include 'serialVMOperations.hpp'
 - fix trialing whitespace
 - ... and 2 more: https://git.openjdk.org/jdk/compare/8f060df7...12c680a3

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/16623/files
  - new: https://git.openjdk.org/jdk/pull/16623/files/0563686a..12c680a3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16623&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16623&range=06-07

  Stats: 627411 lines in 1531 files changed: 93131 ins; 472348 del; 61932 mod
  Patch: https://git.openjdk.org/jdk/pull/16623.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16623/head:pull/16623

PR: https://git.openjdk.org/jdk/pull/16623