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

[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.

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

Commit messages:
 - Replace ShouldNotReachHere by vm_exit_during_initialization

Changes: https://git.openjdk.java.net/jdk/pull/6842/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6842&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278826
  Stats: 5 lines in 4 files changed: 1 ins; 0 del; 4 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