RFR (XS): Add ShenandoahGC into GC flag collision test

[Aleksey Shipilev]

Hi there,

Today is another day I accidentally do -XX:+UseShenandoahGC
-XX:+UseG1GC, and it crashes somewhere in compiler. We should never
allow the combinations like these:
  http://cr.openjdk.java.net/~shade/shenandoah/conflicting-gc-1.patch

Roman, please push?

Thanks,
-Aleksey