RFR: 8278826: Print error if Shenandoah flags are empty (instead of crashing) [v2]

[Dmitry Chuyko]

> ShouldNotReachHere() is replaced by vm_exit_during_initialization() in few places where ShenandoahGCMode and ShenandoahGCHeuristics are examined.
> 
> Generic testing: gc/shenandoah jtreg tests.

Dmitry Chuyko has updated the pull request incrementally with one additional commit since the last revision:

  Flipped ShenandoahGCHeuristics null check

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6842/files
  - new: https://git.openjdk.java.net/jdk/pull/6842/files/5823d5dc..eb9dbf7b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6842&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6842&range=00-01

  Stats: 48 lines in 3 files changed: 18 ins; 21 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6842.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6842/head:pull/6842

PR: https://git.openjdk.java.net/jdk/pull/6842