RFR: DRAFT: Expand old on demand [v27]

[Kelvin Nilsen]

> This PR describes several proposed changes to dynamically adjust the sizes of old-gen and young-gen.  In general, the objective is to keep old-gen as small as possible so that there is an abundance of memory available for the young-gen allocation runway.
> 
> As currently drafted, there are regression failures.  This DRAFT PR is published for the purpose of facilitating a careful code review.

Kelvin Nilsen has updated the pull request incrementally with four additional commits since the last revision:

 - Fix ShouldNotReachHere() assert
 - Remove unnecessary conditional compile directives
 - Remove unnecessary assert
   
   The assert was added at some point during development, but found not to be relevant.
 - Better code reuse for clearing old-gen triggers

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

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/248/files
  - new: https://git.openjdk.org/shenandoah/pull/248/files/c485c006..d8f69909

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=248&range=26
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=248&range=25-26

  Stats: 21 lines in 4 files changed: 0 ins; 17 del; 4 mod
  Patch: https://git.openjdk.org/shenandoah/pull/248.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/248/head:pull/248

PR: https://git.openjdk.org/shenandoah/pull/248