RFR: Fix full gc with class unloading [v2]

[Kelvin Nilsen]

> This pull request fixes incompatibilities between Full GC and class unloading.  In particular, when degenerated GC upgrades to Full GC, it is important to change the generation to GLOBAL.  Otherwise, certain assertions related to class unloading fail.

Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:

  Respond to feedback: remove unneeded code
  
  Replaced with an assertion.

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

Changes:
  - all: https://git.openjdk.java.net/shenandoah/pull/67/files
  - new: https://git.openjdk.java.net/shenandoah/pull/67/files/fc33f30b..82e1e6e0

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=67&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=67&range=00-01

  Stats: 8 lines in 1 file changed: 0 ins; 4 del; 4 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/67.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/67/head:pull/67

PR: https://git.openjdk.java.net/shenandoah/pull/67