RFR (S) Remove SystemDictionary::InitOption enum

[Ioi Lam]

https://bugs.openjdk.java.net/browse/JDK-8212642
http://cr.openjdk.java.net/~iklam/jdk12/8212642-remove-sysdict-initoption.v01/

SystemDictionary::InitOption is no longer needed because we don't support
multiple version of the JDK class libraries anymore. All classes preloaded
by SystemDictionary must exist.

The JVMCI classes were declared using InitOption::Jvmci, so that they 
are loaded
only when EnableJVMCI==true. However, this is rather convoluted and
I just added a few null checks instead.

Tested using hs tiers 1-5

Thanks
- Ioi