RFR: JDK-8314777: GenShen: Alias young and old marking bits to legacy Shenandoah marking bit in gc state [v4]

[Kelvin Nilsen]

> This change sets the MARKING bit of _gc_state whenever either Young marking or Old marking is active.  With this change, barrier code more closely resembles the code in the original single-generation Shenandoah.  The performance impact is negligible.  The primary benefit is to simplify code reviews and clarify that the addition of generational mode to Shenandoah does not negatively impact performance of single-generation Shenandoah.

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

 - Fix up some assert syntax errors
 - Clean up assertions to make them more readable and maintainable

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/309/files
  - new: https://git.openjdk.org/shenandoah/pull/309/files/d3ecbc91..4faf7f67

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=309&range=03
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=309&range=02-03

  Stats: 40 lines in 5 files changed: 21 ins; 10 del; 9 mod
  Patch: https://git.openjdk.org/shenandoah/pull/309.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/309/head:pull/309

PR: https://git.openjdk.org/shenandoah/pull/309