Add a closed set of concrete compiler classes. Using an tag like this avoids a confusing use of macros around the definition of the is_<compiler type> methods.

Here is the change in the Graal repository:

http://hg.openjdk.java.net/graal/graal/diff/5d0bb7d52783/src/share/vm/compiler/abstractCompiler.hpp

We should push this change upstream sans the Graal type.