RFR: JDK-8169872 incremental CMS always crashes VM

[Andrey Petushkov]

Dear All,

Could you please review the simple patch to fix the incremental CMS. Apparently the necessary check was forgotten at the fast path during backporting from jdk9, since the latter does not include iCMS

The problem could easily be reproduced by starting any Java application with the VM supplied the -Xincgc flag

issue: https://bugs.openjdk.java.net/browse/JDK-8169872
webrev: http://cr.openjdk.java.net/~apetushkov/8169872/webrev/

Thanks,
Andrey